Specifically we add
* `gamma(a, b)`, which returns the magnitude of largest 1-ULP
step in range [a, b].
* `count_equidistant_float(a, b, distance)`, which returns the
number of equi-distant floats in range [a, b].
This is a simplification of the fix proposed in #2152, with the
critical function split out so that it can be tested directly,
without having to go through the ULP matcher.
Closes#2152