[bisect] Generalize halving logic in util.rs

Many strategies may resort to a 1-dimensional bisection strategy at some
phase of their execution. This CL centralizes that code by adding a new
"halve_dimension" utility function that accepts the dimension to halve
as a parameter, so it can be used in different situations.

Additionally, this introduces the corrected midpoint logic favoring the
lower half using `(len.saturating_sub(1)) / 2` into `halve_dimension`
and `search_space.rs`.

Bug: 487354432
Change-Id: Id831093c5963c720a3dca1cc5db8bd22d73255ee
Reviewed-on: https://fuchsia-review.googlesource.com/c/fuchsia/+/1550181
Reviewed-by: Aidan Wolter <awolter@google.com>
Commit-Queue: Jerry Belton <jcecil@google.com>