Move the higher-level senv procedures into syntax-environments.

Move the higher-level senv procedures into syntax-environments.

* Clean up the high-level interface, making arg order consistent.
* Rename syntactic-environment->environment as syntactic-environment->runtime.
* Eliminate ->syntactic-environment in favor of runtime-environment->syntactic.
* Rename syntactic-environment/top-level? to top-level-syntactic-environment?.
* Export closed-identifier? to (runtime syntax).