refactor: remove abi submodule

[baszalmstra]

Removes the abi ffi submodule and places the generated ABI header in the repository itself.

[codecov]

Codecov Report

Merging #404 (b22b281) into main (f719ea6) will increase coverage by 0.00%.
The diff coverage is 100.00%.

@@           Coverage Diff           @@
##             main     #404   +/-   ##
=======================================
  Coverage   82.68%   82.68%           
=======================================
  Files         264      264           
  Lines       15239    15239           
=======================================
+ Hits        12600    12601    +1     
+ Misses       2639     2638    -1     

Impacted Files	Coverage Δ
crates/tools/src/abi.rs	100.00% <100.00%> (ø)
crates/mun_hir/src/ty/infer/type_variable.rs	81.25% <0.00%> (+0.89%)	⬆️

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