Add item predicate traits to slices, following `CharEq` for str

[aturon]

Several methods on the str type take a generic "predicate" on characters:

• split and friends
• trim_chars and friends
• find and friends

The CharEq trait makes character predicates convenient by providing instances for char, various char -> bool function types, and slices of characters.

Vector slices should provide a similar interface. Right now, some of the relevant slice methods operate on individual items, while others take bool-producing functions.

Unfortunately, this is blocked on trait reform (#5527).

[huonw]

It's unclear to me if this is possible to implement to get exactly the same sort of API as str, since there element type is very generic, and so impls overlap, e.g.:

impl<T: Eq> ElemEq for T { ... }
impl<F: FnMut(&T) -> bool> ElemEq for F { ... }

[aturon]

Yes, this is a pretty old issue and filed before the final overlapping rules became clear. (I think it might have been workable before unboxed closures, and could perhaps be made to work with negative bounds, but @Kimundi's approach is nicer in any case).