Implement value stack and use it in the interpreter.

We can now run simple programs that don't use function calls.

jsrun.h

@@ -1,6 +1,7 @@
 #ifndef js_run_h
 #define js_run_h
 
+void jsR_error(js_State *J, const char *message);
 void jsR_runfunction(js_State *J, js_Function *F);
 
 #endif