Split code into a Core/Frontend model

[LuizZak]

The idea is to allow future frontends to convert code from other sources into Swift using the existing pipelines in SwiftRewriter.

[codecov[bot]]

Codecov Report

Merging #60 (1d8cadb) into master (0affb4e) will decrease coverage by 6.47%. The diff coverage is 87.04%.

@@            Coverage Diff             @@
##           master      #60      +/-   ##
==========================================
- Coverage   92.03%   85.55%   -6.48%     
==========================================
  Files         433      318     -115     
  Lines       56683    29386   -27297     
==========================================
- Hits        52166    25142   -27024     
+ Misses       4517     4244     -273     

Impacted Files	Coverage Δ
...rces/Core/Analysis/ControlFlowGraph+Creation.swift	97.27% <ø> (ø)
Sources/Core/Analysis/ControlFlowGraph.swift	82.92% <ø> (ø)
Sources/Core/Analysis/DirectedGraph.swift	39.13% <ø> (ø)
...ces/Core/Analysis/ReachingDefinitionAnalyzer.swift	95.17% <ø> (ø)
Sources/Core/Analysis/UsageAnalysis.swift	74.41% <ø> (ø)
...urces/Core/Commons/ArgumentRewritingStrategy.swift	76.47% <ø> (ø)
Sources/Core/Commons/CompoundedMappingType.swift	94.59% <ø> (ø)
...urces/Core/Commons/CompoundedMappingTypeList.swift	100.00% <ø> (ø)
...urces/Core/Commons/CoreGraphicsCompoundTypes.swift	100.00% <ø> (ø)
Sources/Core/Commons/FoundationCompoundTypes.swift	97.30% <ø> (ø)
... and 454 more

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