This commit 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.
[breaking-change]
004db80afe