Return structured information from parsing a crate

[Fluxie]

The more structured type information in the generation stage enables e.g. proper const modifiers for the C++ interfaces.

Output format specific conversion logic was moved to appropriate generators.

[codecov-io]

Codecov Report

Merging #57 into master will decrease coverage by 0.11%. The diff coverage is 80.78%.

@@            Coverage Diff             @@
##           master      #57      +/-   ##
==========================================
- Coverage   76.49%   76.37%   -0.12%     
==========================================
  Files          47       47              
  Lines        2944     3069     +125     
==========================================
+ Hits         2252     2344      +92     
- Misses        692      725      +33

Impacted Files	Coverage Δ
intercom-common/src/foreign_ty.rs	70.31% <71.02%> (-9.96%)	:arrow_down:
intercom-common/src/generators/idl.rs	82.23% <91.48%> (+0.67%)	:arrow_up:
intercom-common/src/generators/cpp.rs	80.23% <91.83%> (+1.04%)	:arrow_up:

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[Rantanen]

Thanks! <3