SL-16094: Extend stringize() to support variadic arguments.

It's useful to be able to say STRINGIZE(item0 << item1 << item2), and we use
that a lot in our code base. But weird syntax aside, there are a couple
advantages to being able to write stringize(item0, item1, item2).

First, it allows stringize() to be used from within some other variadic
function, without having to make that function a macro that accepts an
arbitrary insertion-operator expression. There's no such thing as a member
macro.

Second, particularly for variadic functions, it allows us to optimize the
single-argument case stringize(item0). A macro can't do that. When item0 is
already a string of the desired char type, instead of streaming it into a
std::ostringstream and retrieving it again, we can simply return the input
string. When it's a pointer to the desired char type, we can directly
construct the result string without the help of std::ostringstream. When it's
a string of some other char type, we can engage ll_convert() to perform needed
conversions.

We generalize and optimize the generic gstringize() function, retaining the
role of stringize() and wstringize() as thin wrappers that merely provide the
desired char type.

Optimizing the single-argument case requires separately defining gstringize()
with two or more arguments: the general case. Then gstringize(arg) is
delegated to a gstringize_impl class template so we can partially specialize
to recognize a std::basic_string<desired_char_type> argument, as well as
desired_char_type*. Both these specializations engage ll_convert(), which
already handles the trivial case when no conversion is required.

Use of ll_convert() in this role supercedes and generalizes the previous
wstring_to_utf8str() and utf8str_to_wstring() overloads.

Also introduce stream_to(std::ostream&, ...) to support variadic streaming to
other destinations, e.g. a file, std::cout, ...

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