Handle new Context creation API on the FFI

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.

src/stock.rs

@@ -237,7 +237,6 @@ mod tests {
     use super::*;
     use crate::test_utils::*;
 
-
     use crate::constants::DC_CONTACT_ID_SELF;
     use crate::types::uintptr_t;