Iterate over bit ranges

Sometimes we want to find the ranges of 1 bits, and there's no obviously efficient way to do that currently.
(Maybe I'm missing something!).

If #22 was implemented, then we could do it without allocation, but currently
the only way I can think of doing it with reasonable efficiency (avoiding O(nbits)
complexity I think) is by using Flip and two iterators.

Providing a way to iterator over the unset bits or providing a way to iterate
over the bit-ranges would make this more efficient and/or convenient.

Something like this, perhaps:

// Ranges returns an iterator over all the [start, end)
// ranges of 1-bits in b.
func (b *Bitmap) Ranges() iter.Seq2[uint64, uint64]

Given that, it's trivial to write a function to iterate over the inverted bits.

PoC of the range functions using Flip: https://go.dev/play/p/Ss3T8NCDdJ0

Start by reading the existing Bitmap and Flip behavior, then run the linked Go playground proof of concept to understand the proposed range iteration. Define completion as an iterator that yields each [start, end) range of 1-bits and supports the stated inverted-bit use case without relying on an O(nbits) scan.