refactor: remove internal ffi module

[molpopgen]

This PR removes the internal ffi module,
which had been used to wrap the low-level C
types.

It is no longer clear that we have a robust
pattern warranting a trait.
So we are removing the internal trait until
such a pattern becomes clear.

The issues are:

1. Some C types require malloc to be used properly.
   The table collection is the main example here:
   we need to malloc it so that a tree sequence
   can steal its pointer.
2. Some uses of some C types require uninitialized
   structures.
   This requirement leads to rust code that looks
   sketchy at first glance.
   This PR refactors that code to leave things
   in either a NonNull or a MaybeUninit as long
   as possible.