Multifile Compilation

[ghaith]

There was a regression where multi-files were not compiled correctly anymore
This fixes the issue and adds tests (both unit and integration) to test multifile projects
It also adds test methods that could be used to simulate multiple files during compilation
I also fixed an issue where the correctness tests would sometimes crash with an illegal access exception.
To avoid that, there is now a MainType declared in the tests/tests.rs which can be used when the content of the test struct does not matter. It is currently 1000xusize.

[codecov-commenter]

Codecov Report

Merging #382 (98a6eb9) into master (b173527) will increase coverage by 0.07%.
The diff coverage is 100.00%.

@@            Coverage Diff             @@
##           master     #382      +/-   ##
==========================================
+ Coverage   93.17%   93.24%   +0.07%     
==========================================
  Files          30       31       +1     
  Lines        9933     9979      +46     
==========================================
+ Hits         9255     9305      +50     
+ Misses        678      674       -4     

Impacted Files	Coverage Δ
src/codegen.rs	100.00% <100.00%> (ø)
src/codegen/generators/data_type_generator.rs	95.25% <100.00%> (ø)
src/codegen/generators/pou_generator.rs	91.76% <100.00%> (ø)
src/codegen/generators/variable_generator.rs	98.36% <100.00%> (ø)
src/codegen/llvm_index.rs	97.41% <100.00%> (+0.44%)	⬆️
src/lib.rs	96.01% <100.00%> (+0.17%)	⬆️
src/resolver.rs	96.75% <100.00%> (+0.39%)	⬆️
src/test_utils.rs	100.00% <100.00%> (ø)
src/tests/multi_files.rs	100.00% <100.00%> (ø)
... and 1 more

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[riederm]

cool