Implement Mask::invert()

[kabasset]

Or anything better named, e.g. operator-() for new instance, or match bitset wording.

[kabasset]

bitset provides flip() which is ambiguous wrt. flipping the ordering.
Operators ! or ~ seem appropriate for new instances. One would write:

for (const auto& p : not mask) ...
for (const auto& p : !mask) ...
for (const auto& p : ~mask) ...

[kabasset]

bitset uses operator~() while valarray<bool> uses both while seems to favor operator!