Fixtype of string literals [C:6.4.5] [C:A.1.6].
This is a product type introduced by fty::defprod.
This corresponds to string-literal in the grammar in [C].
The list of natural numbers corresponds to s-char-sequence. As explained in abstract-syntax, these natural numbers represent Unicode code points. We do not capture here the requirement that these characters are not new-line, backslash, and double quote.