Modify various power 2 calculations to use new helper functions

 Modify various power 2 calculations to use new helper functions
 
 First pass of modifying various places that obtain the next power of 2 of
 a number and make them use the new functions added in pg_bitutils.h
 instead.
 
 This also removes the _hash_log2() function. There are no longer any
 callers in core. Other users can swap their _hash_log2(n) call to make use
 of pg_ceil_log2_32(n).
 
 Author: David Fetter, with some minor adjustments by me
 Reviewed-by: John Naylor, Jesse Zhang
 Discussion: https://postgr.es/m/20200114173553.GE32763%40fetter.org