Provide fast path in snprintf.c for conversion specs that are just "%s".

Provide fast path in snprintf.c for conversion specs that are just "%s".

This case occurs often enough (around 45% of conversion specs executed
in our regression tests are just "%s") that it's worth an extra test
per conversion spec to allow skipping all the logic associated with
field widths and padding when it happens.

Discussion: https://postgr.es/m/26193.1538582367@sss.pgh.pa.us