Treat numbers as strings when accessing properties

With this change, numbers are now treated as strings when setting
properties of javascript objects. This aligns with how object properties
are accessed in javascript already:

```sh
$ node
Welcome to Node.js v18.7.0.
Type ".help" for more information.
> var o = {}
undefined
> o[111] = 222
222
> o["111"]
222
>
```

Before this, the following error would be thrown.

```
CL-USER> (jscl::oset 222 (jscl::new) 111)
ERROR: activechars.join is not a function
CL-USER>
```

src/prelude.js

@@ -224,6 +224,7 @@ internals.safe_char_downcase = function(x) {
 };
 
 internals.xstring = function(x){
+  if(typeof x === "number") return x.toString();
   const hasFillPointer = typeof x.fillpointer === 'number'
   const activechars = hasFillPointer? x.slice(0, x.fillpointer): x
   return activechars.join('');

tests/ffi.lisp

@@ -58,4 +58,11 @@
   (test (js-object-p obj))
   (test (js-object-signature obj)))
 
+;;; test in can handle numbers
+
+(let ((obj (make-new #j:Object)))
+  (test (js-object-p obj))
+  (test (oset 456 obj 123))
+  (test (equal 456 (oget obj 123))))
+
 ;;; EOF