And rename IsolateState to ThreadSafeState.

Also make ThreadSafeState directly implement Dispatch. This is simpler.

Reduces generics.

* Refactored the way worker polling is scheduled and errors are handled.
* Share the worker future as a Shared

This simplifies the Behavior trait and makes it more explicit where the
resolve callback is being made.

Also s/StartupScript/Script

To better distinguish the deno_core crate from the executable deno,
which will now be called "the cli" internally.

This disables a few tests which are broken still:
- tests/error_004_missing_module.test
- tests/error_005_missing_dynamic_import.test
- tests/error_006_import_ext_failure.test
- repl_test test_set_timeout
- repl_test test_async_op
- repl_test test_set_timeout_interlaced
- all of permission_prompt_test