move count_bits_set to pwrutisl, put lowest_bit alias there, implement Tagset::tag_size and Tagset::tag_is_singular

Libcorpus2 now contains the base strructures (tokens etc), their I/O and related classes.
Libpwrutils (name TBD) contains basic building blocks common to all tagger-related libraries
Tagset-tool only needs libcorpus, so was brough here as well.