feat(runtime): add marshalling of value structs

[Wodann]

  struct(value) Foo {
      x: int,
      y: int
  }

  pub fn foo_new(x: int, y: int): Foo {
      Foo { x: x, y: y, }
  }

  pub fn foo_add(foo: Foo): int { foo.x + foo.y }

  fn main() {
      let lib_dir = env::args().nth(1).expect("Expected path to a Mun library.");
      let mut runtime = RuntimeBuilder::new(lib_dir)
          .spawn()
          .expect("Failed to spawn Runtime");

      // The Mun compiler automatically generates a wrapper for `pub fn`s that
      // contain `struct(value)` arguments or return types, which allocates the
      // marshalled data in the runtime's allocator.
      let mut foo: StructRef = invoke_fn!(runtime, "foo_new", 1i64, 2i64).wait();
      let x = foo.get::<i64>("x").unwrap();
      let y = foo.replace("y", x + 1).unwrap();
      foo.set("x", y).wait();
  }

@baszalmstra Can you check whether my approach is valid?

What is a good way to implement the generation of the wrapper? Should it be a copy of the body of the statically linked version, or should it just dereference the struct(value) parameters and pass it to the original function?

[codecov]

Codecov Report

Merging #93 into master will increase coverage by 0.39%.
The diff coverage is 92.08%.

@@            Coverage Diff             @@
##           master      #93      +/-   ##
==========================================
+ Coverage   76.82%   77.22%   +0.39%     
==========================================
  Files         122      124       +2     
  Lines        9601     9816     +215     
==========================================
+ Hits         7376     7580     +204     
- Misses       2225     2236      +11     

Impacted Files	Coverage Δ
crates/mun_runtime/src/macros.rs	71.69% <0.00%> (-0.53%)	⬇️
crates/mun_codegen/src/ir/module.rs	100.00% <0.00%> (ø)
crates/mun_runtime/src/function.rs	97.43% <0.00%> (ø)
crates/mun_codegen/src/lib.rs	100.00% <0.00%> (ø)
crates/mun_codegen/src/ir/body.rs	90.39% <0.00%> (+0.75%)	⬆️
crates/mun_codegen/src/ir.rs	55.17% <0.00%> (+5.17%)	⬆️
crates/mun_hir/src/ty.rs	90.99% <0.00%> (+2.52%)	⬆️
crates/mun_codegen/src/type_info.rs	65.00% <0.00%> (+5.38%)	⬆️
crates/mun_runtime/src/struct.rs	88.63% <0.00%> (+8.33%)	⬆️

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[baszalmstra]

I would say we call the original function in the wrapper/bridge function.

[Wodann]

Write wrapper for value structs or stack allocated structs