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- The typeclass `PureExec e1 e2` controls pure deterministic
  reductions
- General tactics rel_pure_l and rel_pure_r that depend on that typeclass
- The same typeclass can be used for WP tactics and tp tactics, potentially

- Use the type of literals in `val`
- Notation for `match`
- "Better" coercions

- Add more binary ops to the language
- An example refinement for module types

Modify the logical relation judment to include an environment Δ that
contains a list of semantic types to interpret free type variables.

- Still does not compile

- Change the types in the examples slightly
- Use notations in the examples
- Modify some tactics to make the proofs more smooth

- This allows us to get rid of `rewrite !Closed_subst_id.`

- Twiggle with the burden of proof in log_related_arrow
- rel_rec_l/r returns a goal with an (unsimplified) lamsubst
  and can detect locked lambdas
- Fix some notational issues [hax.v]
- Rewrite the counter refinement proof to make greater use of lamsubst

We have to call tac_rel_rec_r in reshape_expr because reshape_expr
just decomposes the expression into a `(K',e')` without guaranteeing
that `e` is actually head-reducible. Because of that it might be the
case that `e` is `K[App (Rec f x e1) e2]`, but `e2` is not a value.