pgsql: Allow underscores in integer and numeric constants.

Allow underscores in integer and numeric constants.

This allows underscores to be used in integer and numeric literals,
and their corresponding type input functions, for visual grouping.
For example:

1_500_000_000
3.14159_26535_89793
0xffff_ffff
0b_1001_0001

A single underscore is allowed between any 2 digits, or immediately
after the base prefix indicator of non-decimal integers, per SQL:202x
draft.

Peter Eisentraut and Dean Rasheed

Discussion: https://postgr.es/m/84aae844-dc55-a4be-86d9-4f0fa405cc97%40enterprisedb.com

Branch
------
master

Details
-------
https://git.postgresql.org/pg/commitdiff/faff8f8e47f18c7d589453e2e0d841d2bd96c1ac

Modified Files
--------------
doc/src/sgml/syntax.sgml | 36 ++--
src/backend/catalog/sql_features.txt | 1 +
src/backend/parser/parse_node.c | 43 +---
src/backend/parser/scan.l | 27 +--
src/backend/utils/adt/numeric.c | 106 +++++++---
src/backend/utils/adt/numutils.c | 273 ++++++++++++++++++++------
src/fe_utils/psqlscan.l | 16 +-
src/interfaces/ecpg/preproc/pgc.l | 16 +-
src/pl/plpgsql/src/expected/plpgsql_trap.out | 2 +-
src/pl/plpgsql/src/sql/plpgsql_trap.sql | 2 +-
src/test/regress/expected/int2.out | 44 +++++
src/test/regress/expected/int4.out | 44 +++++
src/test/regress/expected/int8.out | 44 +++++
src/test/regress/expected/numeric.out | 62 +++++-
src/test/regress/expected/numerology.out | 92 ++++++++-
src/test/regress/expected/partition_prune.out | 6 +-
src/test/regress/sql/int2.sql | 14 ++
src/test/regress/sql/int4.sql | 14 ++
src/test/regress/sql/int8.sql | 14 ++
src/test/regress/sql/numeric.sql | 22 ++-
src/test/regress/sql/numerology.sql | 24 ++-
src/test/regress/sql/partition_prune.sql | 6 +-
22 files changed, 724 insertions(+), 184 deletions(-)