* rewrite test runner in Rust
* migrate "test" and "runTests" functions from std to "Deno" namespace
* use "Deno.test()" to run internal JS unit tests
* remove std downloads for Deno subcommands

* Use PathBuf for DenoSubcommand::Bundle's out_file
* Use PathBuf for DenoSubcommand::Format's files
* Use PathBuf for DenoSubcommand::Install's dir
* Use PathBuf for read/write whitelists

* 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

Fixes: #3925

deno builds successfully with VS2019, so no need to require VS2017.

* rename ThreadSafeState to State
* State stores InnerState wrapped in Rc and RefCell

In denoland/deno#2335 a conditional was added to make sure
toAsyncIterator didn't skip chunks because the reader returned data and
EOF in a single call, fixing #2330.

Later, in denoland/deno#2591, the `Reader` interface changed to
`Promise<number | EOF>`. Since the reader no longer returns data and EOF
in a single call, this conditional is not necessary. We can just return
`{ done: true }` when we get `EOF`.
Co-authored-by: NArun Srinivasan <rulfzid@gmail.com>
Co-authored-by: NArun Srinivasan <rulfzid@gmail.com>

- Exports diagnostic items from `diagnostics.ts` which are missing at
  runtime.
- Returns an array of diagnostics, instead of an object with a property
  of `items`.  This is because of the way Rust deals with certain
  structures, and shouldn't be exposed in the APIs.

simplify

Removes OP_HOST_GET_WORKER_LOADED, OP_HOST_POLL_WORKER, 
OP_HOST_RESUME_WORKER and ready/messageBuffer in cli/js/workers.ts.

This change simplifies how we execute V8. Previously V8 Isolates jumped
around threads every time they were woken up. This was overly complex and
potentially hurting performance in a myriad ways. Now isolates run on
their own dedicated thread and never move.

- blocking_json spawns a thread and does not use a thread pool
- op_host_poll_worker and op_host_resume_worker are non-operational
- removes Worker::get_message and Worker::post_message
- ThreadSafeState::workers table contains WorkerChannel entries instead
  of actual Worker instances.
- MainWorker and CompilerWorker are no longer Futures.
- The multi-threaded version of deno_core_http_bench was removed.
- AyncOps no longer need to be Send + Sync

This PR is very large and several tests were disabled to speed
integration:
- installer_test_local_module_run
- installer_test_remote_module_run
- _015_duplicate_parallel_import
- _026_workers