This adopts the FFI API to use the new Context::new() and
Contest::drop() Rust-style APIs while preserving the semantics of the
existing FFI API. It introduces a wrapper around the context which
keeps an optional reference to the actual context, changing dc_open()
and dc_close() to create/destroy the actual reference.
This changes the events emitted on closing slightly: they only get
emitted when dc_close() was called while the context was open. While
dc_close() remains idempotent in that it can still be called safely
multiple times, the close events will only occur if the context is
actually closed.
A new context is now created by calling Context::new and therefore you
always have a valid context. This is much more in Rust style and will
allow a lot of furture simplifications on the context itself.
The FFI layer has not yet been adjusted in this commit and thus will
fail.
* refactor(contact): rename and rusty memory allocations
* refactor(contact): use enum to indidcate origin
* refactor(contact): safe blocking and unblocking api
* refactor(contact): only safe and no more cstrings
