pgsql: Support negative indexes in split_part().

Support negative indexes in split_part().

This provides a handy way to get, say, the last field of the string.
Use of a negative index in this way has precedent in the nearby
left() and right() functions.

The implementation scans the string twice when N < -1, but it seems
likely that N = -1 will be the huge majority of actual use cases,
so I'm not really excited about adding complexity to avoid that.

Nikhil Benesch, reviewed by Jacob Champion; cosmetic tweakage by me

Discussion: https://postgr.es/m/cbb7f861-6162-3a51-9823-97bc3aa0b638@gmail.com

Branch
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master

Details
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https://git.postgresql.org/pg/commitdiff/ec0294fb2c8287fd673c57701cdcf6c6396b2f60

Modified Files
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doc/src/sgml/func.sgml                |  8 +++-
src/backend/utils/adt/varlena.c       | 83 ++++++++++++++++++++++++++++-------
src/test/regress/expected/strings.out | 74 ++++++++++++++++++++++++++++++-
src/test/regress/sql/strings.sql      | 24 ++++++++++
4 files changed, 172 insertions(+), 17 deletions(-)