Add recursion-scheme-friendly ABTs

[sellout]

Without all the associated machinery, we want something like this (but less hand-wavy):

sealed abstract class ABT[F[_], A]
final case class Var[F[_], A](v: String) extends ABT[F, A]
final case class Abs[F[_], A](v: String, term: F[A]) extends ABT[F, A]

type BoundExpr[F[_], A] = HCoproduct(ABT, Expr, F, A)

which relies on us having mutual recursion, but gives us multi-sorted ABTs pretty simply, I think.

[jdegoes]

And maybe with a module-level existential encoding of v so users cannot synthesize their own variable identifiers out of thin air.

[sellout]

Yeah, I actually had a different encoding of vars, but thought it cluttered up the hand-waving.