Add escaped backslash to strings

[bocchino]

Otherwise we can't write the character sequence \q, because it's interpreted as a double-quote character.

[bocchino]

Also think about reverting the notation for an escaped quotation mark to \". We switched to \q because it is less awkward at the end of a string. However, in either case we can't avoid \\, and \" is more standard.

[bocchino]

I've decided to keep \q for quotation marks and add \b for backslashes.

• Keeping \q lets us parse multiline strings in a straightforward way, using the first occurrence of """ as the end delimiter. If we allow \", then with this straightforward approach """Hello\"""" is an illegal character sequence ("""Hello\""" followed by "). To allow this string, we'd have to pass over the """ after the \. To do this, we would need parsing rules with lookahead.

• Keeping \b lets us use simple substitution: (1) apply \q -> " then (2) apply \b -> \. If we use \\ instead, then substitution yields either \\q -> \q -> " or \\q -> \", when in fact the answer should be \q. To make this work, again we would need parsing rules with lookahead.

[bocchino]

After all that, I switched to a state-based parser for strings, instead of a regex-based parser. So I can now support \" and \\. I think it's better because it's more standard.

The new rule is that \c is interpreted as the literal character c in all cases. Nice and simple. In particular, to get a backslash in the interpreted string, you have to write \\ in all cases. This fosters consistency.

[bocchino]

Done in commit 5b60cf7a65d0bf23f8cf31fa2e8a4cf037057174.