feat: memory mapping

[Wodann]

When a struct's memory layout changes, we want any existing instances of that struct to be converted to the new memory layout. E.g. given a struct definition:

struct Foo {
    a: int,
    b: float,
    c: float,
}

we construct a Foo { a: 1, b: 3.0, c: 5.0 }. Now we make several changes to the struct definition, as follows:

struct Foo {
    b: float,   // move
    d: int,     // move + rename
//  c: float,   // remove    
    e: int,     // add
}

For all moved fields, we expect the values to be mapped (i.e. copied), whereas new values should be zero initialized. In the example, resulting struct would look like this:

Foo {
    b: 3.0,  // copied from b
    d: 1,    // copied from a
    e: 0,    // zero initialized
}

The current implementation does not support type casting such as fn(f32) -> f64 yet.

PR #140 adds type casting of primitive types.

[codecov]

Codecov Report

Merging #117 into master will increase coverage by 0.66%.
The diff coverage is 89.91%.

@@            Coverage Diff             @@
##           master     #117      +/-   ##
==========================================
+ Coverage   82.72%   83.39%   +0.66%     
==========================================
  Files         147      158      +11     
  Lines        9704    10646     +942     
==========================================
+ Hits         8028     8878     +850     
- Misses       1676     1768      +92     

Impacted Files	Coverage Δ
crates/mun_abi/src/autogen.rs	64.61% <ø> (ø)
crates/mun_codegen/src/code_gen/linker.rs	23.17% <0.00%> (-2.17%)	⬇️
crates/mun_memory/src/gc.rs	100.00% <ø> (ø)
crates/mun_memory/src/gc/ptr.rs	100.00% <ø> (ø)
crates/mun_memory/src/gc/root_ptr.rs	63.15% <ø> (ø)
crates/mun_memory/tests/gc/alloc.rs	100.00% <ø> (ø)
crates/mun_memory/tests/gc/structs.rs	100.00% <ø> (ø)
crates/mun_memory/tests/gc/util.rs	100.00% <ø> (ø)
crates/mun_runtime/src/function.rs	100.00% <ø> (ø)
crates/mun_runtime/tests/marshalling.rs	100.00% <ø> (ø)
... and 38 more

---

Continue to review full report at Codecov.

Legend - Click here to learn more
Δ = absolute <relative> (impact), ø = not affected, ? = missing data
Powered by Codecov. Last update 18425e7...cd68924. Read the comment docs.

[baszalmstra]

This is one crazy PR. I love it! This seems very promising (given the tests). I'm very curious about how good this works in practice!

👏 👏 for this PR. Amazing job!

I've really only got some minor comments.

[Wodann]

Memory mapping