pgsql: Import changes from IMath versions (1.3, 1.29].

Import changes from IMath versions (1.3, 1.29].

Upstream fixed bugs over the years, but none are confirmed to have
affected pgcrypto.  We're better off naively tracking upstream than
reactively maintaining a twelve-year-old snapshot of upstream.  Add a
header comment describing the synchronization procedure.  Discard use of
INVERT_COMPARE_RESULT(); the domain of the comparisons in question is
{-1,0,1}, controlled entirely by code in imath.c.

Andrew Gierth reviewed the Makefile change.  Tom Lane reviewed the
synchronization procedure description.

Discussion: https://postgr.es/m/20190203035704.GA6226@rfd.leadboat.com

Branch
------
master

Details
-------
https://git.postgresql.org/pg/commitdiff/48e24ba6b7fd3bfd156b51e8d768fd48df0d288b

Modified Files
--------------
contrib/pgcrypto/Makefile |    3 +
contrib/pgcrypto/imath.c  | 2638 ++++++++++++++++++++++-----------------------
contrib/pgcrypto/imath.h  |  493 ++++++---
3 files changed, 1633 insertions(+), 1501 deletions(-)