Rework conversation data types and make safe wrappers. This removes the old `Conversation` type and reworks the FFI types used for PAM conversations. This creates safe `TestResponse` and `BinaryResponse` structures in `conv`, providing a safe way to pass response messages to PAM Conversations. The internals of these types are allocated on the C heap, as required by PAM. We also remove the Conversation struct, which was specific to the real PAM implementation so that we can introduce a better abstraction. Also splits a new `PamApplicationHandle` trait from `PamHandle`, for the parts of a PAM handle that are specific to the application side of a PAM transaction.

350579171e1ff1cef0f1ec1d649cb6df29218952 v0.0.1
bc5e18fa9c7a661b70bc73bdca3520a16cb2ebbc v0.0.2
52f1102e0715e92afeda3c3d859c1cb94a23c4d5 v0.0.3
2a5c83d04b93ef27b1c59d52a0aff46febd920ab v0.0.4
bbe84835d6db43d15b831fbec1c914139c3d8eaf v0.0.5
71e432a213ee099a94655464845c10cdc2e84034 v0.0.6