feat(comb): Make 'fold' a specialization of 'repeat'

[epage]

This follows our pattern of how you process the data is in a trailing method (compared to the grammar which is a free function).

Our only folds are for repeat and separated. For separated, we also aren't providing ranged versions so the Repeat::fold pattern should be sufficient to guide work on that, so I think its ok to close the issue.

Fixes #441

[coveralls]

Pull Request Test Coverage Report for Build 7731048800

• -10 of 34 (70.59%) changed or added relevant lines in 1 file are covered.
• No unchanged relevant lines lost coverage.
• Overall coverage increased (+0.03%) to 44.138%

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Changes Missing Coverage	Covered Lines	Changed/Added Lines	%
src/combinator/multi.rs	24	34	70.59%
<!--	Total:	24	34	70.59%	-->

Totals
Change from base Build 7671180384:	0.03%
Covered Lines:	1314
Relevant Lines:	2977

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💛 - Coveralls