Handle single quotes correctly in the lexer.

[skvadrik]

As mentioned in the comments to https://github.com/skvadrik/re2c/issues/450, different languages use standalone single quotes for different purposes:

• Lisp uses both things like '(a b c) and 'a (both values that are treated as unevaluated constants)
• ML uses both 'a (type variable) and sometimes 'a' (character constant.)
• Haskell uses identifiers like x'
• Rust uses labels and lifetime variables with standalone single quotes

All theses cases should be handled by the lexer, so that it doesn't assume that it's the beginning of a string literal.

[skvadrik]

Instead of handling each possible case in every language, re2c takes a different approach. It only needs to make sure that a closing brace is not part of a single-quoted char literal or string, so when it encounters a single quote in semantic action, it looks for a string (if the language allows single-quoted strings) or a char literal. If it fails to find one of those, the single quote is ignored. Previously re2c also checked if standalone single quotes are allowed by the language, but there's no need for this check, as re2c does not attempt to parse and validate user code.