misc(runtime): add buoyancy example

[Wodann]

Adds an example that showcases a Mun app state setup, struct hot reloading, and struct marshalling.

[codecov]

Codecov Report

Merging #174 into master will decrease coverage by 0.10%.
The diff coverage is n/a.

@@            Coverage Diff             @@
##           master     #174      +/-   ##
==========================================
- Coverage   82.87%   82.76%   -0.11%     
==========================================
  Files         163      163              
  Lines       11267    11240      -27     
==========================================
- Hits         9337     9303      -34     
- Misses       1930     1937       +7     

Impacted Files	Coverage Δ
crates/mun_runtime/src/assembly.rs	62.76% <0.00%> (-8.67%)	⬇️
crates/mun_abi/src/autogen_impl.rs	93.01% <0.00%> (-2.50%)	⬇️
crates/mun_runtime_capi/src/tests.rs	99.47% <0.00%> (-0.02%)	⬇️
crates/mun_runtime/tests/marshalling.rs	99.48% <0.00%> (+0.50%)	⬆️
crates/mun_runtime/src/macros.rs	73.21% <0.00%> (+3.57%)	⬆️
crates/mun_runtime_capi/src/lib.rs	96.77% <0.00%> (+11.24%)	⬆️

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