Add skipbits and index to decodeiterator

[markmentovai]

skipbits makes it possible to skip a portion of a bitarray that does not participate in the prefix coding.

index can be used to determine the position of an iterator within its associated bitarray.

Motivation: I’m decoding a bit stream that’s prefix-encoded, but with each symbol aligned to a 2-bit boundary. I introduced decodeiterator.index to determine the current position within the bit stream, and decodeiterator.skipbits to advance the iterator to the start of the next symbol. skipbits also returns the skipped-over bits so that I can validate that all bits used for alignment are 0.

I exposed the decodeiterator class to provide a clean way to document the new method and data descriptor. Strictly speaking, decodeiterator doesn’t need to be exposed in this way, but that left me wondering how to best provide its documentation.

Documentation and tests are present, and tests pass.

[markmentovai]

Updated. #include “structmember.h” was missing, and this is necessary to make PyMemberDef (used for decodeiterator.index) available in Python < 3.12.

I’ve tested this with 3.8 now.