- `bitstring` started depending on `bitarray` in version 4.1 [0]
- that would mean one additional dependency for us (from yet another maintainer), which is not even pure python
- we only use bitstring for bolt11-parsing
- hence this PR rewrites the bolt11-parsing and removes `bitstring` as dependency
- note: I benchmarked lndecode using [1], and the new code performs better,
taking around 80% time needed for old code (when using bitstring 3.1.9, pure python).
Though the variance is quite large in both cases.
[0]: 95ee533ee4/release_notes.txt (L108)
[1]: d7597d96d0
