* add "assertOps" test assertion which makes sure test case
is not "leaking" ops - ie. after test finishes there are no
pending async ops
* apply "assertOps" to all tests in "cli/js/"
* fix numerous tests leaking ops
* document problem with edge case in "clearInterval"
and "clearTimeout" implementation where they
may leak async ops
* move "cli/js/worker_test.ts" to "cli/tests/worker_test.ts" and
run as integration test; workers leak ops because of missing
"terminate" implementation
* establish basic event loop for workers
* make "self.close()" inside worker
* remove "runWorkerMessageLoop() - instead manually call global function
in Rust when message arrives. This is done in preparation for structured clone
* refactor "WorkerChannel" and use distinct structs for internal
and external channels; "WorkerChannelsInternal" and "WorkerHandle"
* move "State.worker_channels_internal" to "Worker.internal_channels"
* add "WorkerEvent" enum for child->host communication;
currently "Message(Buf)" and "Error(ErrBox)" variants are supported
* add tests for nested workers
* add tests for worker throwing error on startup
