perf(decode) make selecting jws parts faster

[omsmith]

• avoid searching/splitting the input string many times
• 10-20% faster hmac verifications on small inputs

micro-optimizing surely, but it's a pretty small space, and might as well limit the impact as much as possible if you ask me

Runs of jws.verify(jwsWithSig, 'hs256', 'lolol'); goes from ~115kHz to ~136kHz on my laptop.