Report unused global variables

[quchen]

• Warn on unused globals
• Remove unused bindings so they can be GC'd

[quchen]

There is only one scoping rule, and that is that inside a lambda form, the explicit free/bound names and the top-level names are in scope. But bookkeeping over multiple nested closures and lets might be a bit error-prone.

[quchen]

WIP in the static-analysis branch