dynamic_bitset: rfind_first/next, blocks access

add to dynamic_bitset: rfind_first rfind_next (msb instead of lsb - 'rfind' for
reverse of find), blocks(), blocks_end(), block(i) - for efficient external
serialization, hash_value, and other bit-parallelizable algorithms.

add docs and tests

optional compiler intrinsics count leading / trailing zeros instead of
integer_log2 (see boost/pending/lowest_bit)