binary_search takes a haystack, a size, a needle and a compare function.
The compare function should return negative if a < b, positive if a > b
and 0 if a == b. The "sane default" compare function is integral_compare
which implements this with subtraction a - b.
binary_search returns a pointer to a matching element, NOT necessarily
the FIRST matching element. It returns a nullptr if the element was not
found.
This patch includes tests for binary_search.
