Generic constructor for ExecuteFunction

[gumb0]

This is one way to simplify host function definitions, allowing to skip host_context parameter and using lambdas with captures.

Another way could be having another host function pointer type like HostFunctionWithoutContextPtr suggested in #716 (review) (it would not allow lambdas with captures.)

But I think leaving C++ API with none of these might be the least confusing, and most consistent with C API.

[codecov]

Codecov Report

Merging #740 (fbdb105) into master (a83bfb3) will increase coverage by 0.00%.
The diff coverage is 91.37%.

@@           Coverage Diff           @@
##           master     #740   +/-   ##
=======================================
  Coverage   99.25%   99.25%           
=======================================
  Files          76       75    -1     
  Lines       11574    11593   +19     
=======================================
+ Hits        11488    11507   +19     
  Misses         86       86           

Flag	Coverage Δ
rust	99.86% <ø> (ø)
spectests	90.54% <100.00%> (ø)
unittests	99.21% <91.37%> (+<0.01%)	⬆️

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

Impacted Files	Coverage Δ
test/utils/fizzy_engine.cpp	100.00% <ø> (ø)
tools/wasi/wasi.cpp	59.57% <16.66%> (ø)
lib/fizzy/instantiate.hpp	100.00% <100.00%> (ø)
test/unittests/api_test.cpp	100.00% <100.00%> (ø)
test/unittests/execute_call_test.cpp	100.00% <100.00%> (ø)
test/unittests/execute_control_test.cpp	99.46% <100.00%> (ø)
test/unittests/execute_test.cpp	100.00% <100.00%> (ø)
test/unittests/instantiate_test.cpp	100.00% <100.00%> (ø)

[axic]

@gumb0 do we still want this? It is mostly an improvement for testing since the C++ API is not public.

[gumb0]

@gumb0 do we still want this? It is mostly an improvement for testing since the C++ API is not public.

I don't see much benefit, will close it.