feat: add support for extern functions without a return type

[baszalmstra]

This PR fixes an issue where it was not possible to add an extern function like this:

extern "C" fn foo(a: i64) {
    println!("{}", a);
}

because it doesn't have a return type and HasStaticTypeInfo is not implemented for ().

The PR fixes this by adding an implementation of IntoFunctionInfo for functions without a return type.

[codecov]

Codecov Report

Merging #127 into master will increase coverage by 0.03%.
The diff coverage is 100.00%.

@@            Coverage Diff             @@
##           master     #127      +/-   ##
==========================================
+ Coverage   78.81%   78.85%   +0.03%     
==========================================
  Files         141      141              
  Lines        8739     8750      +11     
==========================================
+ Hits         6888     6900      +12     
+ Misses       1851     1850       -1     

Impacted Files	Coverage Δ
crates/mun_runtime/src/function.rs	100.00% <100.00%> (+6.25%)	⬆️
crates/mun_runtime/src/test.rs	98.29% <100.00%> (+0.02%)	⬆️

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