style: Add an owned slice type which cbindgen can understand.

Passing these by value won't be ok of course, but that's fine.

I plan to combine this with https://github.com/eqrion/cbindgen/pull/333 to
actually be able to share representation for ~all the things, this is just the
first bit.

Box<T>, Atom and Arc<T> will be much easier since cbindgen can understand them
without issues.

It's boxed slices the only ones I should need something like this. I could avoid
it if I rely on Rust's internal representation, which we can per [1], but then I
need to teach cbindgen all about slices, which is generally hard, I think.

[1]: https://github.com/rust-lang/unsafe-code-guidelines/blob/master/reference/src/layout/pointers.md

Differential Revision: https://phabricator.services.mozilla.com/D29768

components/to_shmem/lib.rs

@@ -311,7 +311,7 @@ where
 
 /// Writes all the items in `src` into a slice in the shared memory buffer and
 /// returns a pointer to the slice.
-unsafe fn to_shmem_slice<'a, T, I>(src: I, builder: &mut SharedMemoryBuilder) -> *mut [T]
+pub unsafe fn to_shmem_slice<'a, T, I>(src: I, builder: &mut SharedMemoryBuilder) -> *mut [T]
 where
     T: 'a + ToShmem,
     I: ExactSizeIterator<Item = &'a T>,