feat: extern functions

[baszalmstra]

This PR adds the ability to define and call external functions that must be added to the runtime. E.g.

/// This is a function that does not have an implementation in Mun but instead is linked 
/// dynamically when loading the assembly
extern fn tick_the_system();

fn main() {
  // The extern functions can be called like any other Mun function
  tick_the_system()
}

When creating a runtime the tick_the_system function must be added to the runtime by calling insert_fn.

Restrictions

• Structs as parameters or return values are not yet supported

[codecov]

Codecov Report

Merging #96 into master will decrease coverage by 0.64%.
The diff coverage is 62.67%.

@@            Coverage Diff             @@
##           master      #96      +/-   ##
==========================================
- Coverage   77.22%   76.57%   -0.65%     
==========================================
  Files         124      127       +3     
  Lines        9816    10275     +459     
==========================================
+ Hits         7580     7868     +288     
- Misses       2236     2407     +171     

Impacted Files	Coverage Δ
crates/mun_hir/src/ty/infer/coerce.rs	100.00% <0.00%> (ø)
crates/mun_target/src/lib.rs	100.00% <0.00%> (ø)
crates/mun_hir/src/ty/primitives.rs	39.81% <0.00%> (ø)
crates/mun_hir/src/builtin_type.rs	19.29% <0.00%> (ø)
crates/mun_hir/src/expr.rs	75.05% <0.00%> (+0.20%)	⬆️
crates/mun_runtime/src/macros.rs	72.22% <0.00%> (+0.52%)	⬆️
crates/mun_hir/src/ty/tests.rs	97.50% <0.00%> (+0.97%)	⬆️
crates/mun_hir/src/code_model.rs	76.40% <0.00%> (+1.60%)	⬆️
crates/mun_runtime/src/lib.rs	89.24% <0.00%> (+5.03%)	⬆️
crates/mun_abi/src/autogen_impl.rs	97.62% <0.00%> (+2.76%)	⬆️
... and 2 more

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[Wodann]

Implement extern functions