Add a `race` combinator

[ekmett]

#28 pointed out that we need a race combinator for when (<|>) doesn't do what someone expects. We should do that.

[ekmett]

(This will require us to actually track offsets in the consuming error case)

[mikeplus64]

Any progress on this?

[ekmett]

I haven't looked at it yet.

More or less all what has to happen is a porting of the idea from

https://github.com/ekmett/js-parsers

[ekmett]

https://github.com/ekmett/js-parsers/blob/master/parsers.js#L267

[ekmett]

This requires a change to the way unresumable errors are tracked.

[mikeplus64]

#58 : hack to support this