formatting: support named arguments

[bdeane-intel]

We want to write code something like this:

ct_format<"Hello {world}">(arg<"world">("Earth"));

And the resulting object would be something like this (if a runtime argument):

format_result{"Hello {world}", type_map<>{}, tuple{arg<"world">("Earth")}};

or (if a compile-time argument):

format_result{"Hello Earth", type_map<arg<"world">, {6,11}>{}, tuple{}};

where {6,11} is the extent (begin & end indices) of the "world" argument after interpolation.