By moving to use our `uniform_integer_distribution`, which is
reproducible across different platforms, instead of the stdlib
one which is not, we can provide reproducible results for `float`s
and `double`s. Still no reproducibility for `long double`s, because
those are too different across different platforms.

* Utility for extended mult n x n bits -> 2n bits
* Utility to adapt output from URBG to target (unsigned) integral
  type
* Utility to reorder signed values into unsigned type while keeping
  the order.

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].

Co-authored-by: Martin Hořeňovský <martin.horenovsky@gmail.com>

Technically, the declaration should not have a space between
the quotes and the underscore, because `_foo` is a reserved
identifier, but `""_foo` is not. In general it works, but newer
Clang versions warn about this, because WG21 wants to deprecate
and later remove this form completely.