Add methods to go from a slice iterators to a slice.

[huonw]

A slice iterator is isomorphic to a slice, just with a slightly
different form: storing start and end pointers rather than start pointer
and length. This patch reflects this by making converting between them
as easy as iter.as_slice() (or even iter[] if the shorter lifetime
is ok). That is, slice.iter().as_slice() == slice.

r? @aturon

[rust-highfive]

Warning

• These commits modify unsafe code. Please review it carefully!

[aturon]

This looks good, but I'm curious why you chose not to implement explicit as_slice and as_mut_slice methods on MutItems that work on borrows (i.e. shorter lifetimes, like the notation does)?

Ultimately, if the [] notation sticks around we will probably remove as_slice, so in the long run it's a moot point, but I wanted to get your thinking.

[huonw]

Purely to avoid duplication between the notation and explicit methods.

[aturon]

@huonw Can you add #[experimental] attributes to the trait impls and new methods? Otherwise, r=me.