Revert "WASM: move save file encoding from JS into C."

Now that save files are (even more) officially ASCII, it's perfectly
safe to pass them to JavaScript as UTF-8 strings.

This means that the form in which save files are shipped from C to
JavaScript is the same is the form in which they're shipped from
JavaScript to C.  That allows for doing new things with them, like
writing them to local storage.

This reverts commit f729f51e475ff98d0caf529f0723ef810b1c88ef.

emccpre.js

@@ -363,7 +363,8 @@ function initPuzzle() {
                 "Click to download the "));
             var a = document.createElement("a");
             a.download = "puzzle.sav";
-            a.href = "data:application/octet-stream," + savefile_text;
+            a.href = "data:application/octet-stream," +
+                encodeURIComponent(savefile_text);
             a.appendChild(document.createTextNode("saved-game file"));
             dlg_form.appendChild(a);
             dlg_form.appendChild(document.createTextNode("."));