Overhaul S"" macro

The previous S"" macro was essentially one giant for loop with a helper
function.  When adding support for inline face value interpolation, it
was clear that that approach was unmaintainable.  As a result, the
implementation has been completely rewritten.  The new S"" macro is more
maintainable, featureful, and correct — now with a documented EBNF
grammar and more validation during expansion.