This commits add two new methods to "Deno.core" namespace: "encode" and "decode".
Those methods are bound in Rust to provide a) fast b) generally available of encoding and decoding UTF-8 strings.
Both methods are now used in "cli/js/dispatch_json.ts".
* remove Isolate.current_send_cb_info
* remove DenoBuf
* remove Isolate.shared_ab
* port Isolate.shared_response_buf (last bit not ported from libdeno)
* add some docs for Isolate and EsIsolate
* refactored RecursiveLoad - it was renamed to RecursiveModuleLoad, it does not take ownership of isolate anymore - a struct implementing Stream that yields SourceCodeInfo
* untangled module loading logic between RecursiveLoad and isolate - that logic is encapsulated in EsIsolate and RecursiveModuleLoad, where isolate just consumes modules as they become available - does not require to pass Arc<Mutex<Isolate>> around anymore
* removed EsIsolate.mods_ in favor of Modules and moved them inside EsIsolate
* EsIsolate now requires "loader" argument during construction - struct that implements Loader trait
* rewrite first methods on isolate as async
