Add a registry for stashing hidden javascript objects from C code.
Move non-standard 'gc' and 'print' global functions into main.c example.
Add userdata class.

js_newuserconstructor(J, tag, fun, n) creates a constructor function.
It pops an object from the top of the stack,
registers the object in the registry with the given tag,
and sets it as the constructor function's prototype property.

js_newuserdata(J, tag, ptr) creates a userdata object, wrapping the ptr,
taking the prototype from the registry using the tag as key.