feat: error recovery, handling and mapping

[norskeld]

Right now there're no "errors" per se, i.e. all sigma provides users with is text messages and ability to re-map those messages to something custom. This is a shame and should be improved.

Implementation of spans in #34 should help a bit, but we will also need to extend Parser<T> signature with a second generic parameter E, so parsers could bear error type information. All parsers and combinators will be changed accordingly, although I'm pretty sure there'll be hurdles here and there.

Additionally, there should be added two combinators:

• mapErr(parser, fn) - this combinator will map error from E to some other type using given fn.
• mapOrElse(parser, okFn, errFn) - this combinator will conditionally apply okFn or errFn depending on the parser's result.

It also makes sense to implement error recovery along with the stuff above. Hopefully, it will be enough to provide a single combinator:

• recovery(parser, fn) - this combinator takes a parser and a recovery function fn that should produce another parser; similar to when combinator, but it acts only on failures.

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Making parsers named (adding name property to Parser<T>) wouldn't hurt as well.