Unify int/float `to_string`/`from_string`.
Unify int/float `to_string`/`from_string`.

Completes the move to a single pair of generic templates for converting
built-in numeric to/from strings in C++17.  The instantiations are now
just explicit instantiations of the templates, not a bunch of
specialisations which each individually call the generic implementation.

jtv/libpqxx,jtv/libpqxx,jtv/libpqxx,jtv/libpqxx