This is a pre-requisite for moving the existing `combine`-based parser
to use `rust-peg` -- part of the push to use `rust-peg` for all parsing
started in https://github.com/mozilla/mentat/pull/681. We need the
types for the parsed structure "very early", and the `edn` crate is
the earliest such crate.
This is an unfortunate destruction of boundaries between parts of the
system, but it's the best way we have to achieve this right now.
