misc: enables adding extern functions through c api

[baszalmstra]

Enables adding extern functions to the runtime through the C api

Uses: mun-lang/runtime-ffi#7

[codecov]

Codecov Report

Merging #169 into master will increase coverage by 0.10%.
The diff coverage is 72.54%.

@@            Coverage Diff             @@
##           master     #169      +/-   ##
==========================================
+ Coverage   82.76%   82.87%   +0.10%     
==========================================
  Files         163      163              
  Lines       11240    11267      +27     
==========================================
+ Hits         9303     9337      +34     
+ Misses       1937     1930       -7     

Impacted Files	Coverage Δ
crates/mun_runtime_capi/src/lib.rs	85.52% <43.75%> (-11.25%)	⬇️
crates/mun_abi/src/autogen_impl.rs	95.50% <75.00%> (+2.49%)	⬆️
crates/mun_runtime/tests/marshalling.rs	98.97% <75.00%> (-0.51%)	⬇️
crates/mun_runtime/src/assembly.rs	71.42% <85.71%> (+8.66%)	⬆️
crates/mun_runtime_capi/src/tests.rs	99.48% <100.00%> (+0.01%)	⬆️
crates/mun_runtime/src/macros.rs	69.64% <0.00%> (-3.58%)	⬇️

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[Wodann]

[BEWARE] Before merging, do have a look at the other PR. I had some comments there.