* add bitmap accumulator
refactor vec backend so we can use it outside of tests
introduce a "hash only" vec backend for the accumulator
* get core tests passing
* initial test coverage for bitmap_accumulator
* better test coverage for bitmap accumulator and cleanup code
* refactor txhashset roots, call validate() on roots during block validation
* fix store tests
* log the "merged" root when validating roots
* cleanup, revise based on feedback
* cleanup
* rework it to pass explicit size into bitmap accumulator when applying
* fix: split state validation status into kernel and rproof updates. And fix sync status for these two states
* fix: show correct number of leaves for pruned MMR as well as unpruned
* docs: better docs for kernel/range proof validation
* fix: ordering of kernel and rproofs validation in TUI
* fix: typo in rangeproofs api and comments
* wip
* exhaustive match
* write with fixed v1 strategy when writing for hashing
* local protocol version is 2
* cleanup "size" tests that exercise v1 vs v2 vs default protocol versions
* add proto version to Connected! log msg
* cleanup docs
* negotiate protocol version min(local, peer) when doing hand/shake
* introduce HeaderVersion (was u16) for type safety
cleanup pow ser/deser (version unused)
* fixup tests for HeaderVersion
* validate header version during header deserialization
* expose leaf pos iterator
use it for various things in txhashset when iterating over outputs
* fix
* cleanup
* rebuild output_pos index (and clear it out first) when compacting the chain
* fixup tests
* refactor to match on (output, proof) tuple
* add comments to compact() to explain what is going on.
* get rid of some boxing around the leaf_set iterator
* cleanup
