* remove fetch_source_file_and_maybe_compile_async and 
  replace it with State.fetch_compiled_module

* remove SourceFile.js_source()

* introduce CompiledModule which is basically the same as
  deno::SourceInfo and represents arbitrary file that has been 
  compiled to JS module

* introduce //cli/compilers module containing all compilers

* introduce JsCompiler which is a no-op compiler
  - output is the same as input, no compilation takes place
  - it is used for MediaType::JavaScript and MediaType::Unknown

* introduce JsonCompiler that wraps JSON in default export

* support JS-to-JS compilation using checkJs

* merge SourceFileFetcher trait and FileFetcher struct

* move logic related to source file fetching to //cli/file_fetcher.rs

* use Result when creating new ThreadSafeState

* rename `ModuleMetaData` to `SourceFile` and remove TS specific
  functionality

* add `TsCompiler` struct encapsulating processing of TypeScript files

* move `SourceMapGetter` trait implementation to `//cli/compiler.rs`

* add low-level `DiskCache` API for general purpose caches and use it in
  `DenoDir` and `TsCompiler` for filesystem access

* don't use hash-like filenames for compiled modules, instead use
  metadata file for storing compilation hash

* add `SourceFileCache` for in-process caching of loaded files for fast
  subsequent access

* define `SourceFileFetcher` trait encapsulating loading of local and
  remote files and implement it for `DenoDir`

* define `use_cache` and `no_fetch` flags on `DenoDir` instead of using
  in fetch methods

This is in preperation for dynamic import (#1789), which is more easily
implemented when errors are dynamic.

The rules are now as follows:

* In `import` statements, as mandated by the WHATWG specification,
  the import specifier is always treated as a URL.
  If it is a relative URL, it must start with either / or ./ or ../

* A script name passed to deno as a command line argument may be either
  an absolute URL or a local path.
  - If the name starts with a valid URI scheme followed by a colon, e.g.
    'http:', 'https:', 'file:', 'foo+bar:', it always interpreted as a
    URL (even if Deno doesn't support the indicated protocol).
  - Otherwise, the script name is interpreted as a local path. The local
    path may be relative, and operating system semantics determine how
    it is resolved. Prefixing a relative path with ./ is not required.

A lot of its functionality is unused still, but the goal it to slowly
migrate logging functionality to it. There is also a useful progress bar
which can be ported over later - it depends on this module.

https://github.com/rust-lang/cargo/blob/4c1fa54d10f58d69ac9ff55be68e1b1c25ecb816/src/cargo/util/progress.rs

Adds ModuleSpecifier, which wraps a URL. This is now passed around instead of
specifier and resolver strings.

* 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.

Use `--config`

This is to work around Tokio's panic recovery feature.
Ref https://github.com/tokio-rs/tokio/issues/495
Ref https://github.com/tokio-rs/tokio/issues/209
Ref https://github.com/denoland/deno/issues/1311
Fixes #2097

And rename IsolateState to ThreadSafeState.

Also make ThreadSafeState directly implement Dispatch. This is simpler.

Reduces generics.

* Moves how snapshots are supplied to the Isolate. Previously they were
  given by Behavior::startup_data() but it was only called once at
  startup. It makes more sense (and simplifies Behavior) to pass it to the
  constructor of Isolate.
* Adds new libdeno type deno_snapshot instead of overloading
  deno_buf.
* Adds new libdeno method to delete snapshot deno_snapshot_delete().
* Renames deno_get_snapshot() to deno_snapshot_new().
* Makes StartupData hold references to snapshots. This was implicit when
  it previously held a deno_buf but is made explicit now. Note that
  include_bytes!() returns a &'static [u8] and we want to avoid
  copying that.

And model worker resources as Stream

Fixes #1742
Fixes #2021

* 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