pgsql: Replace binary search in fmgr_isbuiltin with a lookup array.

Replace binary search in fmgr_isbuiltin with a lookup array.

Turns out we have enough functions that the binary search is quite
noticeable in profiles.

Thus have Gen_fmgrtab.pl build a new mapping from a builtin function's
oid to an index in the existing fmgr_builtins array. That keeps the
additional memory usage at a reasonable amount.

Author: Andres Freund, with input from Tom Lane
Discussion: https://postgr.es/m/20170914065128.a5sk7z4xde5uy3ei@alap3.anarazel.de

Branch
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master

Details
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https://git.postgresql.org/pg/commitdiff/212e6f34d55c910505f87438d878698223d9a617

Modified Files
--------------
src/backend/utils/Gen_fmgrtab.pl | 79 +++++++++++++++++++++++++++++++++-------
src/backend/utils/Makefile | 2 +-
src/backend/utils/fmgr/fmgr.c | 29 ++++++---------
src/include/utils/fmgrtab.h | 11 +++++-
4 files changed, 88 insertions(+), 33 deletions(-)