This provides a mechanism to emit literal values in informative
messages, such as query parameters.  The new code is more complex than
what it replaces, primarily because it wants to be more efficient.
It also has the (currently unused) additional optional capability of
specifying a maximum size to print.

The new function lives out of common/stringinfo.c so that frontend users
of that file need not pull in unnecessary multibyte-encoding support
code.

Author: Álvaro Herrera and Alexey Bashtanov, after a suggestion from Andres Freund
Reviewed-by: Tom Lane
Discussion: https://postgr.es/m/20190920203905.xkv5udsd5dxfs6tr@alap3.anarazel.de