calling `clear` on a `bit_array` no longer leaks the previous
allocation, instead it sets all bits to `false` preserving the same
backing dynamic array.
It's now possible to iterate over:
- all keys in the range min_value ..= max_value, with `iterate_all`
- all set keys in the bit array, with `iterate_set`
- all unset keys in the range min_value ..= max_value, with `iterate_unset`
`Bit_Array` now stores the `max_value` provided during construction, and
updates it when a key that was previously out of range is set.
