Fixes #596.

It'd be nice to support each of `optional`, `shared_ptr`, `unique_ptr`,
and potentially other smart pointer types for each of these string
types.  But doing that without C++20's "concepts" is going to be a
little hairy.

Extends special-case support for `string_view` etc. in `stream_to` to
optional values as well.

This is ugly.  I want to do better.  But it looks like that'll work a
lot better with C++20.

Getting some warnings that look wholly unrelated to my changes, and in
fact seem to be completely internal to the library implementation.

I can't reproduce this compile warning in a `debian:testing` container
on my own machine.  So, just in case the problem is a caching effect,
let's try a newer Debian version.