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[bot]]

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

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[WebFreak001]

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