implement bitfield parsing

[WebFreak001]

fixes #464

should get merged soon, because of new DMD release around the corner, so would like a quick review ^^

upstream DMD implementation: https://github.com/dlang/dmd/blob/a7f8c84d405df2c0dd8929d2479c1760f61762e0/compiler/src/dmd/parse.d#L4621

[codecov]

Codecov Report

Merging #476 (4be179e) into master (7348c5f) will increase coverage by 0.05%.
The diff coverage is 100.00%.

Additional details and impacted files

@@            Coverage Diff             @@
##           master     #476      +/-   ##
==========================================
+ Coverage   83.27%   83.33%   +0.05%     
==========================================
  Files          11       11              
  Lines        8378     8402      +24     
==========================================
+ Hits         6977     7002      +25     
+ Misses       1401     1400       -1     

Impacted Files	Coverage Δ
src/dparse/ast.d	70.83% <100.00%> (+0.23%)	⬆️
src/dparse/astprinter.d	94.95% <100.00%> (+0.02%)	⬆️
src/dparse/formatter.d	46.46% <100.00%> (+0.47%)	⬆️
src/dparse/parser.d	91.42% <100.00%> (+0.01%)	⬆️

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[WebFreak001]

gonna merge this because release is nearing (in 5 days)