Should `Constructor` and `AccumConstructor` be newtypes?

[danidiaz]

• It would be more uniform with how other phases are handled (other phases are not "peeled" by the Bare TF).
• Perhaps it would make component insertions into the DI context less confusing.
• Constructor is a simple function, perhaps overkill in this case.
• For AccumConstructor, it may help hide the mandatory lazy pattern-match. Forgetting it causes infinite loops!
• Them being synonyms for Compose is cute for calling fixEnv, but perhaps it shouldn't be exposed.

How to make the transition without an immediate breaking change? Deprecate the types in Dep.Env (including fixEnv and fixEnvAccum and add the new versions in a separate modle Dep.Constructor. This would give us the added bonus of having a natural place to explain how "DI by tying the knot" works.

[danidiaz]

Added in 2c8aef6611a2f395596b75c723c8aca05662615d.