Provide separate header file for built-in float types

Provide separate header file for built-in float types

Some data types under adt/ have separate header files, but most simple
ones do not, and their public functions are defined in builtins.h.  As
the patches improving geometric types will require making additional
functions public, this seems like a good opportunity to create a header
for floats types.

Commit 1acf757255 made _cmp functions public to solve NaN issues locally
for GiST indexes.  This patch reworks it in favour of a more widely
applicable API.  The API uses inline functions, as they are easier to
use compared to macros, and avoid double-evaluation hazards.

Author: Emre Hasegeli
Reviewed-by: Kyotaro Horiguchi
Discussion: https://www.postgresql.org/message-id/CAE2gYzxF7-5djV6-cEvqQu-fNsnt%3DEqbOURx7ZDg%2BVv6ZMTWbg%40mail.gmail.com