Add support for bracket expressions

[yallop]

This PR adds basic support for POSIX bracket expressions such as:

• [a-zA-Z0-9]
• [^\n\r\t]
• [aceghi-z]

Both parsing (parsing a bracket expression as part of a regular expression) and unparsing (turning a set of char into a bracket expression are supported. Both directions involve some care to correctly handle characters that have special meaning either in regular expressions (e.g. ., |, *) or in bracket expressions (], ^, -). For example,

• ^ has a special meaning (complement) at the beginning of a bracket expression, but not elsewhere
• - has a special meaning (range) except at the beginning or end of a bracket expression
• ] has a special meaning (terminate the bracket expression) except at the beginning of a bracket expression (i.e. after the opening [ or [^)

Unparsing prefers shorter output expressions: for example, for the set of all characters except e unparsing returns [^e], not [\000-df-\255].