Implement split_pattern on slices

[eduardorittner]

From #49036, adds a split_pattern method on slices which takes a second slice and splits using that one.
I was unsure about where to add additional tests, specifically to test reverse iteration using the next_back method, my guess would be in library/core/tests/slice.rs, however when looking at the tests I saw no test for unstable features, so I was unsure if I should add them somewhere else.

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[fmease]

Suggested change

	/// Allows splitting a slice by another slice
	(unstable, split_pattern, "CURRENT_RUSTC_VERSION", Some(49036)),

Library features shouldn't (and don't need to) be declared inside the compiler.

[fmease]

Suggested change

split_pattern,

(see above)

[fmease]

This UI test should be removed. It's only required to exist right now by tidy because you declared a library feature as a compiler feature, too. The doctest should be sufficient.

[eduardorittner]

Done! I was following the rustc-dev-guide and thought that all unstable features should be declared in the compiler, thanks for clarifying!

[tgross35]

This should get proposed via an ACP, which is one of the issue templates at https://github.com/rust-lang/libs-team/issues. That gives the libs-api team a chance to figure out if the new API makes sense before you put too much effort into the implementation.

I was unsure about where to add additional tests, specifically to test reverse iteration using the next_back method, my guess would be in library/core/tests/slice.rs, however when looking at the tests I saw no test for unstable features, so I was unsure if I should add them somewhere else.

As fmease mentioned, library features don't really need to touch anything outside of library/, no compiler changes are needed. library/core/tests/iter would be a reasonable place to test this.

[eduardorittner]

Thank you so much for your help I issued an APC at rust-lang/libs-team#457 and will be going from there