pgsql: Speed up byteain by not parsing traditional-style input twice.

Speed up byteain by not parsing traditional-style input twice.

Instead of laboriously computing the exact output length, use strlen
to get an upper bound cheaply. (This is still O(N) of course, but
the constant factor is a lot less.) This will typically result in
overallocating the output datum, but that's of little concern since
it's a short-lived allocation in just about all use-cases.

A simple microbenchmark showed about 40% speedup for long input
strings.

While here, make some cosmetic cleanups and add a test case that
covers the double-backslash code path in byteain and byteaout.

Author: Steven Niu <niushiji(at)gmail(dot)com>
Reviewed-by: Kirill Reshke <reshkekirill(at)gmail(dot)com>
Reviewed-by: Stepan Neretin <slpmcf(at)gmail(dot)com>
Reviewed-by: Tom Lane <tgl(at)sss(dot)pgh(dot)pa(dot)us>
Discussion: https://postgr.es/m/ca315729-140b-426e-81a6-6cd5cfe7ecc5@gmail.com

Branch
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master

Details
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https://git.postgresql.org/pg/commitdiff/3683af617044d271ab7486d43d06f9689ed4961d

Modified Files
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src/backend/utils/adt/bytea.c | 61 +++++++++--------------------------
src/test/regress/expected/strings.out | 12 +++++++
src/test/regress/sql/strings.sql | 2 ++
3 files changed, 30 insertions(+), 45 deletions(-)