Helps with cases like "\xFFecho" by limiting the number of hex digits
that will be parsed to maximum suitable for the char type.
This rule differs from the C/C++ standard, but ChaiScript does not offer
the same workaround options.
Furthermore, without it having hexadecimal sequences longer than can fit
into the char type is undefined behavior anyway.
The parser code just added the first character after an octal/hex sequence
as raw text, resulting in erroneous data whenever another escape
sequence follows directly after.