- the values from non-overlapping blocks were simply ignored,
which meant that ('111111111111' & '111111') would not mask out
the unset values at all.
- similar oddities in other operations (|=, ^= etc)
where the original implementation tried hard to avoid touching the
sizing at all, but now better resolved as follows:
- '|=' : Set may grow to accommodate new 'on' bits.
- '^=' : Set may grow to accommodate new 'on' bits.
- '-=' : Never changes the original set size.
- '&=' : Never changes the original set size.
Non-overlapping elements are considered 'off'.
These definitions are consistent with HashSet behaviour
and also ensures that (a & b) == (b & a)
ENH: improve short-circuiting within bitSet ops
- in a few places can optimise by checking for none() instead of
empty() and avoid unnecessary block operations.
ENH: added bitSet::resize_last() method
- as the name says: resizes to the last bit set.
A friendlier way of writing `resize(find_last()+1)`
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