class probe_seq

The state for a probe sequence.

Currently, the sequence is a triangular progression of the form

p(i) := Width * (i^2 + i)/2 + hash (mod mask + 1)

The use of `Width` ensures that each probe step does not overlap groups;

the sequence effectively outputs the addresses of *groups* (although not

necessarily aligned to any boundary). The `Group` machinery allows us

to check an entire group with minimal branching.

Wrapping around at `mask + 1` is important, but not for the obvious reason.

As described above, the first few entries of the control byte array

are mirrored at the end of the array, which `Group` will find and use

for selecting candidates. However, when those candidates' slots are

actually inspected, there are no corresponding slots for the cloned bytes,

so we need to make sure we've treated those offsets as "wrapping around".

It turns out that this probe sequence visits every group exactly once if the

number of groups is a power of two, since (i^2+i)/2 is a bijection in

Z/(2^m). See https://en.wikipedia.org/wiki/Quadratic_probing