Change advance(_back)_by to return `usize` instead of `Result<(), usize>`

A successful advance is now signalled by returning `0` and other values now represent the remaining number
of steps that couldn't be advanced as opposed to the amount of steps that have been advanced during a partial advance_by.

This simplifies adapters a bit, replacing some `match`/`if` with arithmetic. Whether this is beneficial overall depends
on whether `advance_by` is mostly used as a building-block for other iterator methods and adapters or whether
we also see uses by users where `Result` might be more useful.

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