Implement `From<Cow<str>> for String` and `From<Cow<[T]>> for Vec<T>`.

Motivation: the `selectors` crate is generic over a string type,
in order to support all of `String`, `string_cache::Atom`, and
`gecko_string_cache::Atom`. Multiple trait bounds are used
for the various operations done with these strings.
One of these operations is creating a string (as efficiently as possible,
re-using an existing memory allocation if possible) from `Cow<str>`.

The `std::convert::From` trait seems natural for this, but
the relevant implementation was missing before this PR.
To work around this I’ve added a `FromCowStr` trait in `selectors`,
but with trait coherence that means one of `selectors` or `string_cache`
needs to depend on the other to implement this trait.
Using a trait from `std` would solve this.

The `Vec<T>` implementation is just added for consistency.
I also tried a more general
`impl<'a, O, B: ?Sized + ToOwned<Owned=O>> From<Cow<'a, B>> for O`,
but (the compiler thinks?) it conflicts with `From<T> for T` the impl
(after moving all of `collections::borrow` into `core::borrow`
to work around trait coherence).

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