Introduce new verify and parse interface

[shogo82148]

The current Parse function does not verify the alg, iss, and sub claims. By introducing a new Parser structure, it makes it easier to verify these claims.

[coveralls]

Pull Request Test Coverage Report for Build 6445434741

• 161 of 204 (78.92%) changed or added relevant lines in 4 files are covered.
• No unchanged relevant lines lost coverage.
• Overall coverage increased (+0.03%) to 77.061%

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Changes Missing Coverage	Covered Lines	Changed/Added Lines	%
jws/key_finder.go	7	9	77.78%
jws/verifier.go	41	45	91.11%
jwt/parser.go	112	149	75.17%

Totals
Change from base Build 6430607648:	0.03%
Covered Lines:	7421
Relevant Lines:	9630

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💛 - Coveralls