To better reflect changes in error types in JS from #3662 this PR changes
default error type used in ops from "ErrBox" to "OpError".
"OpError" is a type that can be sent over to JSON; it has all
information needed to construct error in JavaScript. That
made "GetErrorKind" trait useless and so it was removed altogether.
To provide compatibility with previous use of "ErrBox" an implementation of
"From<ErrBox> for OpError" was added, however, it is an escape hatch and
ops implementors should strive to use "OpError" directly.
* 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
