fix(comb): Deprecate top-level opaque impls

[epage]

This is to avoid cluttering up the main docs page with irrelevant information.

We could hide these but then you can't inspect them if needed (e.g. to verify they implement a trait).

I didn't do this to Repeat because it has functionality on it and so is less opaque.

This is a cherry-pick of #673

[coveralls]

Pull Request Test Coverage Report for Build 12640262380

Details

• 0 of 11 (0.0%) changed or added relevant lines in 1 file are covered.
• No unchanged relevant lines lost coverage.
• Overall coverage increased (+0.03%) to 41.469%

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Changes Missing Coverage	Covered Lines	Changed/Added Lines	%
src/parser.rs	0	11	0.0%

Totals
Change from base Build 12607251300:	0.03%
Covered Lines:	1276
Relevant Lines:	3077

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💛 - Coveralls