Rename `slice::take...` methods to `split_off...`
This rename was discussed and recommended in a recent t-libs meeting.
cc https://github.com/rust-lang/rust/issues/62280
There's an additional commit here which modifies internals of unstable `OneSidedRange` APIs in order to implement `split_off` methods in a panic-free way (remove `unreachable!()`) as recommended in https://github.com/rust-lang/rust/pull/88502/files#r760177240. I can split this out into a separate PR if needed.