feat(runtime_capi): add garbage collection methods

[Wodann]

Adds functions necessary for making a full-fledged C++ wrapper around the runtime.

This should contain the last changes to the runtime CAPI before the Mun v0.2 release 🤞

[codecov]

Codecov Report

Merging #163 into master will increase coverage by 0.14%.
The diff coverage is 99.33%.

@@            Coverage Diff             @@
##           master     #163      +/-   ##
==========================================
+ Coverage   82.71%   82.85%   +0.14%     
==========================================
  Files         162      163       +1     
  Lines       11130    11221      +91     
==========================================
+ Hits         9206     9297      +91     
  Misses       1924     1924              

Impacted Files	Coverage Δ
crates/mun_runtime_capi/src/hub.rs	80.95% <ø> (ø)
crates/mun_runtime_capi/src/lib.rs	96.77% <95.23%> (-0.20%)	⬇️
crates/mun_runtime/src/lib.rs	71.87% <100.00%> (ø)
crates/mun_runtime/src/struct_ref.rs	76.80% <100.00%> (ø)
crates/mun_runtime_capi/src/gc.rs	100.00% <100.00%> (ø)
crates/mun_runtime_capi/src/tests.rs	99.47% <100.00%> (+0.18%)	⬆️

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