Constify numeric.c.

This allows the compiler/linker to move the static variables to a
read-only segment.  Not all the signature changes are necessary, but
it seems better to apply const in a consistent manner.

Reviewed-By: Tom Lane
Discussion: https://postgr.es/m/20170910232154.asgml44ji2b7lv3d@alap3.anarazel.de

Author: anarazel