Use simpler impls for some `Iterator` methods for slices.

The default implementations of several `Iterator` methods use `fold` or
`try_fold`, which works, but is overkill for slices and bloats the
amount of LLVM IR generated and consequently hurts compile times.

This commit adds the simple, obvious implementations for `for_each`,
`all`, `any`, `find`, `find_map`, and simplifies the existing
implementations for `position` and `rposition`. These changes reduce
compile times significantly on some benchmarks.

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