Better diagnostic for turbofish syntax

[sbillig]

error[1-0001]: expected newline or `}`
   ┌─ storage_map.fe:24:31
   │
24 │     let mut st = StorageMap::<u256, u256, 0>::new()
   │                               ^ expected newline or `}`

[KiChjang]

Should the correct behaviour be allowing this to parse successfully, or emit an error telling the user to use the PathSegment<GenericArgList> syntax?

[sbillig]

I think it should be an error, or warning at least.. We currently refuse to compile if there are any diagnostics, and there are no warnings yet. Someday we should differentiate, and add more lint-like warnings that don't block compilation, but error would be fine for now.