Use wasmExports instead of obsolete asm.

[darkstarx]

It's related to #10.

New versions of Emscripten declares the Model.asm as deprecated and generates the getter asm which aborts with corresponding message:

legacyModuleProp('asm', 'wasmExports');
...
function legacyModuleProp(prop, newName, incomming=true) {
  if (!Object.getOwnPropertyDescriptor(Module, prop)) {
    Object.defineProperty(Module, prop, {
      configurable: true,
      get() {
        let extra = incomming ? ' (the initial value can be provided on Module, but after startup the value is only looked for on a local variable of that name)' : '';
        abort(`\`Module.${prop}\` has been replaced by \`${newName}\`` + extra);

      }
    });
  }
}
...
missingGlobal('asm', 'Please use wasmExports instead');

So, to keep old versions of Emscripten supported as well as new versions, this PR is offered.