Process source maps in Rust instead of JS (#1280)

- Improves speed and binary size significantly.
- Makes deno_last_exception() output a JSON structure.
- Isolate::execute and Isolate::event_loop now return
  structured, mapped JSError objects on errors.
- Removes libdeno functions:
  libdeno.setGlobalErrorHandler()
  libdeno.setPromiseRejectHandler()
  libdeno.setPromiseErrorExaminer()

In collaboration with Ryan Dahl.

js/libdeno.ts

@@ -1,5 +1,4 @@
 // Copyright 2018 the Deno authors. All rights reserved. MIT license.
-import { RawSourceMap } from "./types";
 import { globalEval } from "./global_eval";
 
 // The libdeno functions are moved so that users can't access them.
@@ -39,9 +38,6 @@ interface Libdeno {
   ) => void;
 
   setPromiseErrorExaminer: (handler: () => boolean) => void;
-
-  mainSource: string;
-  mainSourceMap: RawSourceMap;
 }
 
 const window = globalEval("this");