* 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
