Add fail parser

[cenodis]

This PR introduces a fail parser that works as the inverse of the success parser. It always fails regardless of what input was passed to it. This complements the success parser and allows specifying default cases to return an error instead.

I have come across one use case as part of my precedence parser #1362. It requires three parser for prefix, postfix and binary operators respectively. But some languages might not have any prefix or postfix operators so the respective parser should just fail.

Compared to a workaround like verify(tag(""), |_: &str| false), fail is easier to read and makes the intent more clear when a parser is required (e.g. as part of a parameter) but should just fail because its not needed in the context.

[cenodis]

Its not unrecoverable. Its the normal nom::Err::Error. Though I guess the name fail could be confused with the unrecoverable Failure variant. But I dont want yet another thing called "error" in nom considering that there are already 4 or 5 things with the name "error". Any other suggestions?

[Stargateur]

Well, failure should be ok when we will rename the error type in nom, for now it's probably confusing. Maybe for now forward ? Cause I still wonder how to rename Err::Error to either Failure or Forward;

Or maybe recoverable ? (meh)

[Geal]

naming it fail will be fine for now

[Geal]

thanks!