add `IntoCow` to the prelude
SendStr: change HashMap::find_equiv tests with HashMap::get tests
Add `is_borrowed`/`is_owned` methods to `Cow`
Deprecate MaybeOwnedVector
Deprecate MaybeOwned
impl BorrowFrom<Cow<'a, T, B>> for B
Ignore doctests that broke due to deprecation
Replace MaybeOwned with Cow in libgraphviz docs
Rename `Display::as_maybe_owned` to `as_cow`
Remove IntoMaybeOwned trait from the prelude
Add/fix doc comments on type aliases