create mostly empty baseclass for FFI::Raw::MemPtr

Here is a mostly complete implementation for what we discussed in #20:

  - made `_call_deref` and called it `coderef` so there is a public interface for creating a coderef.  we can rename it.
  - no documentation (yet), wanted to see if you had any concerns or suggestions before writing it up, or if you prefer you can write it.