auto merge of #18638 : aturon/rust/as_slice_dst, r=japaric

This PR changes `AsSlice` to work on unsized types, and changes the
`impl` for `&[T]` to `[T]`. Aside from making the trait more general,
this also helps some ongoing work with method resolution changes.

This is a breaking change: code that uses generics bounded by `AsSlice`
will have to change. In particular, such code previously often took
arguments of type `V` where `V: AsSlice<T>` by value. These should now
be taken by reference:

```rust
fn foo<Sized? V: AsSlice<T>>(v: &V) { .. }
```

A few std lib functions have been changed accordingly.

The PR also relaxes constraints on generics and traits within the
`core::ops` module and for the `Equiv` trait.

[breaking-change]

r? @nikomatsakis 
cc @japaric 

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