feat: add struct as marshallable field type

[Wodann]

Depends on PR #84.

Allows getting, setting and replacing of structs in another struct:

let foo: Struct = some_foo_constructor();
let bar = foo.get<Struct>("bar");
let bar2 = some_bar_constructor();
let old_bar = foo.replace("bar", bar2);
foo.set("bar", old_bar);

[Wodann]

Add marshalling of structs to Rust

[codecov]

Codecov Report

Merging #87 into master will increase coverage by 0.51%.
The diff coverage is 88.37%.

@@            Coverage Diff             @@
##           master      #87      +/-   ##
==========================================
+ Coverage    74.8%   75.32%   +0.51%     
==========================================
  Files         121      121              
  Lines        9599     9613      +14     
==========================================
+ Hits         7181     7241      +60     
+ Misses       2418     2372      -46

Impacted Files	Coverage Δ
crates/mun_runtime/src/reflection.rs	41.5% <100%> (+20.99%)	⬆️
crates/mun_runtime/src/test.rs	96.9% <100%> (+0.25%)	⬆️
crates/mun_runtime/src/struct.rs	80.59% <100%> (+29.46%)	⬆️
crates/mun_abi/src/autogen_impl.rs	94.57% <45.45%> (-1.11%)	⬇️
crates/mun_runtime/src/macros.rs	73.68% <73.33%> (+30.13%)	⬆️
crates/mun_codegen/src/ir/dispatch_table.rs	92.36% <0%> (+1.38%)	⬆️

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[baszalmstra]

Very nicely implemented!

[baszalmstra]

I think at some point we should figure out a way to do some sort of hash lookup. Because this might become slow.