feat: add mut keyword

[mkhan45]

Adds the mut keyword and updates a lot of tests to accomodate for it.

Closes #199.

🎉

[codecov]

Codecov Report

Merging #305 (b8f33ac) into master (e987aa6) will decrease coverage by 0.00%.
The diff coverage is 80.76%.

@@            Coverage Diff             @@
##           master     #305      +/-   ##
==========================================
- Coverage   81.42%   81.41%   -0.01%     
==========================================
  Files         239      240       +1     
  Lines       14026    14073      +47     
==========================================
+ Hits        11420    11457      +37     
- Misses       2606     2616      +10     

Impacted Files	Coverage Δ
crates/mun_codegen/src/test.rs	100.00% <ø> (ø)
crates/mun_compiler/src/diagnostics.rs	100.00% <ø> (ø)
crates/mun_runtime/tests/marshalling.rs	98.20% <ø> (ø)
crates/mun_hir/src/diagnostics.rs	62.17% <66.66%> (+0.12%)	⬆️
crates/mun_hir/src/ty/infer.rs	72.78% <75.00%> (+0.06%)	⬆️
crates/mun_hir/src/ty/infer/mut_bind.rs	75.00% <75.00%> (ø)
crates/mun_codegen/src/ir/body.rs	83.70% <100.00%> (ø)
crates/mun_hir/src/expr.rs	87.71% <100.00%> (+0.05%)	⬆️
crates/mun_hir/src/ty/tests.rs	97.67% <100.00%> (+0.02%)	⬆️
crates/mun_syntax/src/ast/extensions.rs	85.10% <100.00%> (+0.49%)	⬆️
... and 3 more

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[baszalmstra]

Thanks for this great PR! I've added some cosmetic comments but other than that it looks good to me!

[mkhan45]

I didn't mean to close this; I think I updated the wrong branch on my side. I'll fix it tomorrow.