parser: replace closures with currying

[Kijewski]

Rebased on #297.

IMHO the code more readable with fewer closures.

[GuillaumeGomez]

I'm not a big fan of this approach. I like the changes in the second commit but not in the first. Reading closure makes it much easier to both read and actually know what's going on whereas with the Unbound trait, you actually need to go check where this bind method comes from to understand what it's doing.

[Kijewski]

I mean, you can always grep for fn bind. And bind is phrase used in multiple languages for partial-applications. IMHO the invocations are self-explanatory, the current closures aren't.

[GuillaumeGomez]

It's generic, whereas closures are directly part of the winnow API and are easy to update if needed. IMO it's code obfuscation, not really an improvement.