Remove dc_context_unref from Rust API

This removes the dc_context_unref function from the Rust API which was
just an alias for dc_close.  It still exists on the C API where it
makes sure to free the memory.

It also implements Drop for the context which just calls dc_close to
make sure all the memory is freed.  Since you can call dc_close as
many times as you like this ensures that at the Rust level you can't
Drop the struct without releasing the memory.

Finally since memory is now freed by dropping the struct this removes
the #[repr(C)] for the struct.  This struct is fully opaque to the C
API.

examples/repl/main.rs

@@ -456,12 +456,6 @@ fn main_0(args: Vec<String>) -> Result<(), failure::Error> {
     println!("history saved");
     {
         stop_threads(&ctx.read().unwrap());
-
-        unsafe {
-            let mut ctx = ctx.write().unwrap();
-            dc_close(&mut ctx);
-            dc_context_unref(&mut ctx);
-        }
     }
 
     Ok(())