Added apply method (partially).

I have just added both apply(args) and apply() methods. However,
default arguments are not being generated yet (I think this is due to
quasiquote limitations). Besides they are not working fine, because
they have a dependency with `updated`, which is still unimplemented.
Added `get`, `updated` and `modifiables`.

So, we are pretty advanced in order to fully apply macro @builder.