Optimize strip_prefix and strip_suffix with str patterns
As mentioned in https://github.com/rust-lang/rust/issues/67302#issuecomment-585639226.
I'm not sure whether adding these methods to `Pattern` is desirable—but they have default implementations so the change is backwards compatible. Plus it seems like they're slated for wholesale replacement soon anyway? #56345
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Constructing a Searcher in strip_prefix and strip_suffix is
unnecessarily slow when the pattern is a fixed-length string. Add
strip_prefix and strip_suffix methods to the Pattern trait, and add
optimized implementations of these methods in the str implementation.
The old implementation is retained as the default for these methods.
9ee373fd94