properly implement shrink_to_fit.

properly implement shrink_to_fit.

For whatever reason it treated 1 as the minimum number of blocks and would corrupt
itself if it was already at 0 blocks. It also never actualy did a shrink_to_fit,
it just told the underlying bit_vec that it had less initialized blocks.

So all around just a buggy mess. Thankfully although the bit_set code is 'unsafe'
it wasn't ever actually a memory-safety issue. The unsafe here is a reservation
from bit_vec to be more dangerous, but the implementation never actually has any
unsafe code of its own, so this was just state corruption that would crash,
similar to feeding a bad Ord impl into a BTreeMap. Bad, but not a vulnerability.

Fixes #25