Finally add support for finally

I realized my ignorant omission of `finally` in #4, and implemented this today. Please tell me whether I messed something up. By the way, I'm doing the `goto/break/continue` check at runtime, perhaps it should be done at load-time too?

    - FINALLY block allows for writing cleanup code that always runs:
      try { ... } finally { cleanupCode() }
      In this case (and unlike standalone TRY with no CATCH or FINALLY),
      the exception is NOT caught and after executing the FINALLY block,
      a CATCH block is still searched for upwards in the call stack.
    - FINALLY block can also be used with CATCH:
      try { ... } catch var { ... } finally { cleanupCode() }
    - goto and break/continue are blocked from exiting a try..finally block.