fix: Handling of non-ASCII identifiers

Non-ASCII-identifiers where not handled safely:
```
>>> ö : Int
thread 'main' panicked at ~/.cargo/registry/src/index.crates.io-1949cf8c6b5b557f/tokay-0.6.12/src/compiler/ast.rs:1169:42:
byte index 1 is not a char boundary; it is inside 'ö' (bytes 0..2) of `ö`
note: run with `RUST_BACKTRACE=1` environment variable to display a backtrace
```

Author: phorward

src/compiler/ast.rs

@@ -251,7 +251,7 @@ fn traverse_node_value(scope: &Scope, node: &Dict, name: Option<String>) -> ImlV
                                     else {
                                         format!(
                                             "Argument named '{}' invalid; Use a name starting in lower-case, e.g. '{}{}'",
-                                            name, &name[0..1].to_lowercase(), &name[1..]
+                                            name, name.chars().next().unwrap().to_lowercase(), name.chars().skip(1).collect::<String>()
                                         )
                                     }
                             );
@@ -570,7 +570,7 @@ fn traverse_node_lvalue(scope: &Scope, node: &Dict, store: bool, hold: bool) ->
                         Some(_) => {
                             scope.push_error(
                                 traverse_node_offset(node),
-                                format!("Cannot assign to constant '{}'", name),
+                                format!("Cannot assign variable value to constant '{}'", name),
                             );
                             break 'load;
                         }
@@ -591,7 +591,7 @@ fn traverse_node_lvalue(scope: &Scope, node: &Dict, store: bool, hold: bool) ->
                                 scope.push_error(
                                     traverse_node_offset(node),
 
-                                    if &name[0..1] == "_" {
+                                    if name.chars().nth(0) == Some('_') {
                                         format!(
                                             "The variable '{}' is invalid, only constants may start with '_'",
                                             name
@@ -1165,8 +1165,8 @@ fn traverse_node(scope: &Scope, node: &Dict) -> ImlOp {
                     scope.push_error(
                         traverse_node_offset(node),
                         format!(
-                            "Cannot assign to constant '{}' as consumable. Use an identifier starting in upper-case, e.g. '{}{}'",
-                            ident, &ident[0..1].to_uppercase(), &ident[1..]
+                            "Cannot assign consumable to constant '{}'. Use an identifier starting in upper-case, e.g. '{}{}'",
+                            ident, ident.chars().next().unwrap().to_uppercase(),  ident.chars().skip(1).collect::<String>()
                         )
                     );
 
@@ -1177,14 +1177,14 @@ fn traverse_node(scope: &Scope, node: &Dict) -> ImlOp {
                     traverse_node_offset(node),
                     if ident.starts_with("_") {
                         format!(
-                            "Cannot assign to constant '{}', because it must be consumable. Use an identifier not starting with '_'.",
+                            "Cannot assign non-consumable to constant '{}'. Use an identifier not starting with '_'.",
                             ident
                         )
                     }
                     else {
                         format!(
-                            "Cannot assign to constant '{}', because it must be consumable. Use an identifier starting in lower-case, e.g. '{}{}'",
-                            ident, &ident[0..1].to_lowercase(), &ident[1..]
+                            "Cannot assign non-consumable to constant '{}'. Use an identifier starting in lower-case, e.g. '{}{}'",
+                            ident, ident.chars().next().unwrap().to_lowercase(), ident.chars().skip(1).collect::<String>()
                         )
                     }
                 );

tests/compiler_identifiers.tok

@@ -13,21 +13,27 @@ Factorial : factorial  # Error: Cannot assign non-consumable to consumable const
 
 IsOkay : @{
     Int if $1 > 100 && $1 < 1000 accept
-}  # Ok, because the function is a parselet as it calls Cident
+}  # Ok, because the function is a parselet as it calls Ints
 
 _ : "abc"
 
 # https://github.com/tokay-lang/tokay/issues/149
 a : 1
 a = 2
+Ö = 1
 
 b = 3
 b : 4
+ö : Int
+Ä : 123
 
 #---
-#ERR:Line 1, column 1: Cannot assign to constant 'Pi', because it must be consumable. Use an identifier starting in lower-case, e.g. 'pi'
-#ERR:Line 5, column 1: Cannot assign to constant 'cident' as consumable. Use an identifier starting in upper-case, e.g. 'Cident'
-#ERR:Line 12, column 1: Cannot assign to constant 'Factorial', because it must be consumable. Use an identifier starting in lower-case, e.g. 'factorial'
-#ERR:Line 18, column 1: Cannot assign to constant '_', because it must be consumable. Use an identifier not starting with '_'.
-#ERR:Line 22, column 1: Cannot assign to constant 'a'
-#ERR:Line 25, column 1: Cannot assign constant value to variable 'b'
+#ERR:Line 1, column 1: Cannot assign non-consumable to constant 'Pi'. Use an identifier starting in lower-case, e.g. 'pi'
+#ERR:Line 5, column 1: Cannot assign consumable to constant 'cident'. Use an identifier starting in upper-case, e.g. 'Cident'
+#ERR:Line 12, column 1: Cannot assign non-consumable to constant 'Factorial'. Use an identifier starting in lower-case, e.g. 'factorial'
+#ERR:Line 18, column 1: Cannot assign non-consumable to constant '_'. Use an identifier not starting with '_'.
+#ERR:Line 22, column 1: Cannot assign variable value to constant 'a'
+#ERR:Line 23, column 1: Cannot assign variable named 'Ö'; Try lower-case identifier, e.g. 'ö'
+#ERR:Line 26, column 1: Cannot assign constant value to variable 'b'
+#ERR:Line 27, column 1: Cannot assign consumable to constant 'ö'. Use an identifier starting in upper-case, e.g. 'Ö'
+#ERR:Line 28, column 1: Cannot assign non-consumable to constant 'Ä'. Use an identifier starting in lower-case, e.g. 'ä'

tests/str_find.tok

@@ -0,0 +1,20 @@
+#testmode:repl
+s = "hello"
+
+s.find("l",3)
+s.find("l",2)
+s.find("l",1)
+s.find("l",0)
+s.find("l",4)
+s.find("l",3,4)
+s.find("l",3,3)
+s.find("l",3,2)
+#---
+#3
+#2
+#2
+#2
+#-1
+#3
+#-1
+#-1