Allow multiple object files as input

[ghaith]

Fixes #405

Moved the compile and link logic to the lib
Added a --relocatable flag to allow linking object files in an intermediate step
Created separate methods that are now testable independently.
Integration tests for the linker still missing.

[codecov-commenter]

Codecov Report

Merging #419 (9eb3a38) into master (0ba48e3) will decrease coverage by 0.13%.
The diff coverage is 66.20%.

@@            Coverage Diff             @@
##           master     #419      +/-   ##
==========================================
- Coverage   93.21%   93.08%   -0.14%     
==========================================
  Files          31       31              
  Lines       11474    11656     +182     
==========================================
+ Hits        10696    10850     +154     
- Misses        778      806      +28     

Impacted Files	Coverage Δ
src/diagnostics.rs	81.74% <50.00%> (-0.83%)	⬇️
src/linker.rs	75.89% <59.09%> (+30.20%)	⬆️
src/lib.rs	85.41% <65.21%> (-12.38%)	⬇️
src/cli.rs	97.45% <90.47%> (-0.51%)	⬇️

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