Add context-free code getter for ClosureParser

Hi,

This PR adds "context-free" code getter, i.e. closure's code which doesn't depend on context via `$this` and `use (...)`. If it is not possible, getter raises `InvalidArgumentException`.

I suppose it could be useful in case of moving closure's code to somewhere like cache with assurance that we are able to move code without breaking something.

I'm not absolutely sure that it's OK, though.