ENH: Change parsing logic to bytes manipulation directly

[greglucas]

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Speed up parsing by removing bitstring reads

Avoid bitstring and do the bitwise reading and manipulations ourselves with bitwise logic. This greatly increases the performance of the parser by ~3-5x in the read routine. This is sort of the MVP for speed-ups by just implementing the routines we need and trying to replicate bitstring. We could do better by refactoring the _get_format_string() logic and avoiding any string matching/parsing/splitting too.

I did not fully remove bitstring at this point since there is a lot of logic surrounding it in parser.py. We could make it an optional dependency, but it would be nice to get rid of the legacy_parse_packet() method if we go that route. Happy to add that deprecation path here if it'd be useful, but probably a major version bump then.

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Checklist

• [x] Changes are fully implemented without dangling issues or TODO items
• [ ] Deprecated/superseded code is removed or marked with deprecation warning
• [ ] Current dependencies have been properly specified and old dependencies removed
• [n/a] New code/functionality has accompanying tests and any old tests have been updated to match any new assumptions
• [ ] The changelog.md has been updated