Having now the guarantee that VCL sources are always null-terminated we
can use standard string.h functions to detect long strings. We can also
generalize vcc_decstr() to work with all kinds of strings.

Instead of duplicating the code in 3 locations with slight variations,
encode the specificities of each type of string into a struct and have
one location to parse them.

Bonus deduplication, inline C parsing is identical.

This is loosely inspired by vcl_fixed_token() and we could further
reduce the size of the loop in vcc_Lexer() by extracting more logic.