This patch makes it so that RecursiveLoad doesn't own the Isolate, so
Worker::execute_mod_async does not consume itself.

Previously Worker implemented Loader, but now ThreadSafeState does.

This is necessary preparation work for dynamic import (#1789) and import
maps (#1921)

* Compiler no longer has its own Tokio runtime. Compiler handles one
  message and then exits.

* Uses the simpler ts.CompilerHost interface instead of
  ts.LanguageServiceHost.

* avoids recompiling the same module by introducing a hacky but simple
  `hashset<string>` that stores the module names that have been already
  compiled.

* Removes the CompilerConfig op.

* Removes a lot of the mocking stuff in compiler.ts like `this._ts`. It
  is not useful as we don't even have tests.

* Turns off checkJs because it causes fmt_test to die with OOM.

* In order to prevent ArrayBuffers from getting garbage collected by V8,
  we used to store a v8::Persistent<ArrayBuffer> in a map. This patch
  introduces a custom ArrayBuffer allocator which doesn't use Persistent
  handles, but instead stores a pointer to the actual ArrayBuffer data
  alongside with a reference count. Since creating Persistent handles
  has quite a bit of overhead, this change significantly increases
  performance. Various HTTP server benchmarks report about 5-10% more
  requests per second than before.

* Previously the Persistent handle that prevented garbage collection had
  to be released manually, and this wasn't always done, which was
  causing memory leaks. This has been resolved by introducing a new
  `PinnedBuf` type in both Rust and C++ that automatically re-enables
  garbage collection when it goes out of scope.

* Zero-copy buffers are now correctly wrapped in an Option if there is a
  possibility that they're not present. This clears up a correctness
  issue where we were creating zero-length slices from a null pointer,
  which is against the rules.

Use `--config`

Op dispatch is now dynamically dispatched, so slightly less efficient.
The immeasurable perf hit is a reasonable trade for the API simplicity
that is gained here.

Changes "deno --types" to "deno types"
and "deno --prefetch" to "deno prefetch"

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