A simple elems implementation for MVMContext REPR

<jnthn>	lizmat: (.elems) looks like something we should be able to fix.
<lizmat> jnthn: fixing nqp::elems on an MVMContext, would be great

src/6model/reprs/MVMContext.c

@@ -104,8 +104,10 @@ static void bind_key(MVMThreadContext *tc, MVMSTable *st, MVMObject *root, void
 }
 
 static MVMuint64 elems(MVMThreadContext *tc, MVMSTable *st, MVMObject *root, void *data) {
-    MVM_exception_throw_adhoc(tc,
-        "MVMContext representation does not support elems");
+    MVMContextBody *body  = (MVMContextBody *)data;
+    MVMFrame       *frame = body->context;
+    MVMLexicalRegistry *lexical_names = frame->static_info->body.lexical_names;
+    return (MVMuint64) HASH_CNT(hash_handle, lexical_names);
 }
 
 static MVMint64 exists_key(MVMThreadContext *tc, MVMSTable *st, MVMObject *root, void *data, MVMObject *key) {