Improve the slice iterator's searching methods
Improve all, any, find, position, rposition by explicitly unrolling the loop for the slice iterators.
- Introduce a few extension methods and functions for raw pointers make the new code easy to express
- Introduce helper methods `search_while, rsearch_while` that generalize all the searching methods
LLVM doesn't unroll the loop in `.find()` by default (clang is the same), so performance benefits a lot from explicit unrolling here. An iterator method without conditional exits (like `.fold()`) does not need this on the other hand.
One of the raw pointer extension methods is `fn post_inc(&mut self) -> Self` which is the rustic equivalent of “`ptr++`”, and it is a nice way to express the raw pointer loop (see commit 3).
Specific development notes about `search_while`: I tried both computing an end pointer "rounded" to 4, as well as the `ptrdistance >= 4` loop condition, ptrdistance was better. I tried handling the last 0-3 elements unrolled or with a while loop, the loop was better.
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