Make MultiPeek consistent with Peekable

[mysteriouslyseeing]

MultiPeek is missing 2 methods from Peekable: next_if and next_if_eq. This pull request adds those two methods and two additional ones: nth_if and nth_if_eq. This will make MultiPeek a drop-in replacement for Peekable in the standard library.

Description of changes: Added next_if, nth_if, next_if_eq and nth_if_eq.

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[mysteriouslyseeing]

I just realised that this is not enough; peek_mut (and peek_nth_mut) are also required. I'll add them to the PR in the morning.

[mysteriouslyseeing]

I've now added the two methods that were missing