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
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master

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

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