Implement by-reference interning.

[ryzhyk]

This branch implements an optimization whereby we intern values by reference, avoiding cloning the value in the common case that the same value already exists in the interner. Unfortunately, this optimization doesn't work with the internment crate due to the issue described here: https://github.com/droundy/internment/blob/260c9d90ed336dc31212086828de0a5d8f2890ef/src/lib.rs#L521. This is in turn due to the HashMap::get() requiring the key argument to implement Borrow. One workaround is to use the raw HashMap API, which is currently unstable: https://doc.rust-lang.org/std/collections/struct.HashMap.html#method.raw_entry.