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- Have a version of bind that allocates a local variable, and
  automatically frees it at the end.
- Only malloced memory can be freed manually.

- Get rid of fractions in `env_inv`.
- Separate the concerns

- Consistent notation for ↦ of the C language: `x ↦C[lv]{q] v`, which is
  not redefined in every file.
- Many useless stylistic changes.

This implements
- Composable propositions in flock with `flock_res`
- Ability to add resources to AWP "on the fly"

- amonadG Σ
- + clean up a bit

- Add awp_pure to lifting.v
- Prove a_load without unfolding awp
- Use IntoVal in some rules in monad.v