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

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

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