[FFI] Add handle to pointer functions

[JP-Ellis]

Handles have been introduced as a way to avoid passing pointers directly, specifically for languages which don't support pointers nicely. The FFI currently has a mix of functions which require a pointer or handles.

Unfortunately, a number of FFI functions still require a pointer, and do not have a handle equivalent. As one example of the discrepancy:

• pactffi_with_specification takes a Pact handle and updates the specification; but,
• pactffi_pact_spec_version takes a Pact pointer and return the specification.

There's unfortunately no way at present to obtain the pointer of the Pact behind a specific Pact handle.

[rholshausen]

Currently, both the Pact handle and Pact pointer own their content. We can add a function to return a Pact pointer from a Pact handle, but it will be clone of the contents.

[rholshausen]

I've added two functions:

• pactffi_pact_handle_to_pointer: converts a PactHandle to a mut * Pact (via a clone of the inner Pact)
• pactffi_handle_get_pact_spec_version: returns the spec version enum for the PactHandle