doc: clarifying unsafe code vs undefined behavior
auto merge of #19105 : alfie/rust/master, r=thestinger

As discussed in pull #19068, trying to make the wording more clear for unsafe code vs undefined behavior.
libcore: DST-ify AsSlice

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]
libcore: DSTify ops traits, Equiv

This commit relaxes constraints on generics and traits within the
`core::ops` module and for the `Equiv` trait.
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