[PIBD] PMMR Desegmenter Structure (Pt. 1)

[yeastplume]

This is the first part of several anticipated PRs to enable and test the reconstruction of the txhashset from PMMR segments. Note that none of the code introduced here is called from anywhere but the main test test_pibd_copy. Merging this a bit earlier than I'd planned, but breaking up the work like this keeps the review process a bit more manageable, and the included test is needed to help track down a potential serious performance issue in header sync 😄

Included in this PR:

• Introduce a desegmenter type containing functions to validate and append PMMR segments.
• Add optional instance of desegmenter to chain, with lazy init method
• Function in BitmapAccumulator to return the accumulator as a raw bitmap for use in desegmenter validation
• Add functions in txhashset extension to apply outputs, rangeproofs and kernels within an extension
• Add test to copy sample dataset from previous PR #3665 into new txhashset.
• Fix for SegmentIdentifier returning wrong number of segments in count_segments_required
• change last_pos to mmr_size in Segment to be consistent with changes #3666

Note the current copy tests do work, however they haven't been tested or verified against actual pruned data.

[yeastplume]

Thank you, will address bits before merging this. It's perhaps a bit early for review and there are still a lot of things here that are going to change, but I'd just like to get everything I have to date merged before continuing with this.

[yeastplume]

Addressed a few issues, will keep the rest in consideration during future work