feat(combinator): try_fold and verify_fold

[Altair-Bueno]

Closes #99

Implementation notes

Not sure if the documentation examples are good enough. In particular, I couldn't find any (non-nightly) simple examples for showcasing try_fold. Should we add something a bit more verbose or keep things simple?

[coveralls]

Pull Request Test Coverage Report for Build 12715974807

Details

• 0 of 85 (0.0%) changed or added relevant lines in 1 file are covered.
• 1 unchanged line in 1 file lost coverage.
• Overall coverage decreased (-1.1%) to 40.278%

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Changes Missing Coverage	Covered Lines	Changed/Added Lines	%
src/combinator/multi.rs	0	85	0.0%

Files with Coverage Reduction	New Missed Lines	%
src/token/mod.rs	1	75.0%

Totals
Change from base Build 12702423757:	-1.1%
Covered Lines:	1274
Relevant Lines:	3163

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💛 - Coveralls

[epage]

btw I'm generally fine having commits squashed during review and much prefer the commits to match user-facing changes (so a fix meaning something in master is fixed, not fixed within the same PR)