There are many areas in the code where we need to determine the next
highest power of 2 of a given number.  We tend to always do that in an
ad-hoc way each time, generally with some tight for loop which performs a
bitshift left once per loop and goes until it finds a number above the
given number.

Here we add two generic functions which make use of the existing
pg_leftmost_one_pos* functions which, when available, will allow us to
calculate the next power of 2 without any looping.

Here we don't add any code which uses these new functions. That will be
done in follow-up commits.

Author: David Fetter, with some minor adjustments by me
Reviewed-by: John Naylor, Jesse Zhang
Discussion: https://postgr.es/m/20200114173553.GE32763%40fetter.org