Introduce direct-mapping types for `string' and `float array'.
Merge branch 'whitequark-master'
Fix raw tests after changes to internal interfaces.
Update stub generation after pointer representation change.
No need to special-case pointers in write_arg.

The catch-all case has the same behaviour.
Pull out the selection of ctypes_call/ctypes_call_errno.
Sketch of some loop unrolling for libffi calls on the OCaml side.