[query] Intern values in query parser

It's currently difficult for us to add any kind of interning to the EDN parser: see https://github.com/kevinmehall/rust-peg/issues/84.

However, in #395 I'm about to make Variable and TypedValue wrappers around an Rc. We thus have an opportunity to intern query parts within the query parser itself, even if the EDN value stream itself contains duplicate strings

This has some immediate value: not only do we get cloneable ConjoiningClauses (and other consumers of Variable and TypedValue) — the point of #395 — but also as we drop the repeated [edn::Value] parser inputs we can prune some memory.

To do this involves maintaining state for the duration of our combine parse: probably a little struct around a few InternSet<PlainSymbol> and InternSet<String> instances.

This would be threaded into the top parser (Find::find), and then down into each parser it creates. I think the simplest way to do that — avoiding lifetime and mutability issues — is to wrap our interner in an Rc and pass it by cloning. (It's theoretically possible to use a ThreadLocal for this, but global state is a bit of a downer.)

We'd discard the interner when we're done with the parse. A future optimization is to keep it around….