Pass staticScope through interpreter call stack + optimized imports.

* In preparation for a fix for #1792, pass the current static scope
  through the interpreter call stack.

* Unrelated: I accidentally applied optimize imports on the ir/
  directory while refactoring in IntelliJ.

* Next steps:
  - Make the nesting-depth fix outlined in #1792.
  - Maybe, rename CurrentScope to ActiveScope.
  - Maybe, fix ScopeModule to be based off CurrentScope.
  - Maybe, get rid of currDynScope arg through call stack and get it from
    ThreadContext where needed (maybe).
Fix #1792: Remove IRScope references from CurrentScope and ScopeModule

* Record nesting depth and use that to traverse the static-scope
  hierarchy.

* Persistence code is not fully fixed up -- will require fixup when
  it is time to revive that code.

* Some other unrelated cleanup.

* No change in rubyspecs and mri test results.