capi: Remove dynamic allocation in find_exported_function

[gumb0]

This is a PoC to get rid of extra dynamic allocation in C API for imported/exported functions at the expense of

• some changes/complications in API (members added to FizzyExternalFunction, but the upside is that fizzy_free_exported_function is removed)
• not being able to access C++ host functions, only C host functions available on C side.
• some C API details leaking into C++ side.

[codecov]

Codecov Report

Merging #791 (fd6ade7) into master (2ea01bc) will decrease coverage by 0.06%.
The diff coverage is 85.71%.

@@            Coverage Diff             @@
##           master     #791      +/-   ##
==========================================
- Coverage   99.23%   99.16%   -0.07%     
==========================================
  Files          79       79              
  Lines       12394    12396       +2     
==========================================
- Hits        12299    12293       -6     
- Misses         95      103       +8     

Flag	Coverage Δ
rust	99.90% <ø> (ø)
spectests	90.26% <0.00%> (-0.29%)	⬇️
unittests	99.09% <85.71%> (-0.08%)	⬇️

Flags with carried forward coverage won't be shown. Click here to find out more.

Impacted Files	Coverage Δ
lib/fizzy/capi.cpp	95.17% <66.66%> (-2.39%)	⬇️
lib/fizzy/instantiate.hpp	96.55% <85.71%> (-3.45%)	⬇️
lib/fizzy/instantiate.cpp	100.00% <100.00%> (ø)
test/unittests/capi_test.cpp	99.89% <100.00%> (+<0.01%)	⬆️
test/utils/fizzy_c_engine.cpp	96.66% <100.00%> (ø)