webassembly: Make mp_js_do_str asynchronous.

This fixes a bug where `gc.collect()` would crash due to
emscripten_scan_stack being called synchronously within mp_js_do_str.  The
fix is to make mp_js_do_str asynchronous.

Fixes micropython#10692.

Signed-off-by: Eli Bierman <eli@elib.dev>

ports/webassembly/README.md

@@ -49,7 +49,7 @@ using the require command and the general API outlined below. For example:
 var mp_js = require('./build/micropython.js');
 
 mp_js_init(64 * 1024);
-mp_js_do_str("print('hello world')\n");
+await mp_js_do_str("print('hello world')\n");
 ```
 
 Running with HTML
@@ -77,7 +77,7 @@ something to stdout.  The following code demonstrates basic functionality:
       Module["onRuntimeInitialized"] = async function() {
         mp_js_startup();
         mp_js_init(64 * 1024);
-        mp_js_do_str("print('hello world')");
+        await mp_js_do_str("print('hello world')");
       };
     </script>
   </body>
@@ -108,7 +108,7 @@ Initialize MicroPython with the given stack size in bytes. This must be
 called before attempting to interact with MicroPython.
 
 ```
-mp_js_do_str(code)
+await mp_js_do_str(code)
 ```
 
 Execute the input code. `code` must be a `string`.

ports/webassembly/wrapper.js

@@ -29,7 +29,7 @@ var Module = {};
 var mainProgram = function()
 {
   mp_js_init = Module.cwrap('mp_js_init', 'null', ['number']);
-  mp_js_do_str = Module.cwrap('mp_js_do_str', 'number', ['string']);
+  mp_js_do_str = Module.cwrap('mp_js_do_str', 'number', ['string'], {async: true});
   mp_js_init_repl = Module.cwrap('mp_js_init_repl', 'null', ['null']);
   mp_js_process_char = Module.cwrap('mp_js_process_char', 'number', ['number']);
 
@@ -75,7 +75,9 @@ var mainProgram = function()
               }
           });
       } else {
-          process.exitCode = mp_js_do_str(contents);
+          mp_js_do_str(contents).then(exitCode => {
+              process.exitCode = exitCode
+          })
       }
   }
 }