frontier: tighter integration with rust traits

[petrosagg]

The frontier module implements a common Rust pattern where there is
one type representing a mutable version of something and a separate type
representing its immutable counterpart. Examples in std are String and
str, PathBuf and Path, OsString and OsStr, and other.

In all the examples above the mutable version holds onto a heap
allocated buffer and the immutable version is just a
#[repr(transparent)] wrapper over an raw unsized slice of the data
type.

This patch changes AntichainRef<T> to simply be a transparent wrapper
over [T]. This change enables Deref implementations targeting the
immutable antichain from both MutableAntichain<T> and Antichain<T>.

This has a bunch of ergonomic improvements for users (due to auto-deref)
and also allows having a single implementation of all methods that only
need immutable access to the antichain.

Specifically, the following methods have been deduplicated and are now
implemented in only once place:

• AntichainRef::less_than
• AntichainRef::less_equal
• AntichainRef::dominates
• AntichainRef::elements
• <AntichainRef as PartialEq>::eq
• <AntichainRef as PartialOrd>::partial_cmp
• <AntichainRef as Hash>::hash

Finally, this change also enables replacing the inherent
Antichain::borrow and Antichain::to_owned methods by implementing
the std::borrow::Borrow and std::borrow::ToOwned traits
respectively.

Signed-off-by: Petros Angelatos petrosagg@gmail.com