Return match position

[marmeladema]

sliceslice currently only returns a boolean to denote if the substring has been found or not.
Most crate are actually providing the match position.
It would be nice to offer a similar API, but without sacrificing performances.
This should help to ensure correctness (do we find the right needle at the right position?) but also improve the fairness of our benchmarks since we currently actually compare with crates that are returning strictly more information.