Remove dc_open() from the Rust API

This means the Context becomes a struct following the normal Rust
conventions where when it is created it is usable.  This will over
time allow removing a lot of runtime checks and simplify code.  Many
members will no longer need to be Options or similar.

The C API needs to remain compatible so hides the implementation of
this behind another struct which can be opened and closed.

python/tests/test_lowlevel.py

@@ -17,11 +17,19 @@ def test_callback_None2int():
     clear_context_callback(ctx)
 
 
-def test_dc_close_events():
-    ctx = capi.lib.dc_context_new(capi.lib.py_dc_callback, ffi.NULL, ffi.NULL)
+def test_dc_close_events(tmpdir):
+    ctx = ffi.gc(
+        capi.lib.dc_context_new(capi.lib.py_dc_callback, ffi.NULL, ffi.NULL),
+        lib.dc_context_unref,
+    )
     evlog = EventLogger(ctx)
     evlog.set_timeout(5)
-    set_context_callback(ctx, lambda ctx, evt_name, data1, data2: evlog(evt_name, data1, data2))
+    set_context_callback(
+        ctx,
+        lambda ctx, evt_name, data1, data2: evlog(evt_name, data1, data2),
+    )
+    p = tmpdir.join("hello.db")
+    lib.dc_open(ctx, p.strpath.encode("ascii"), ffi.NULL)
     capi.lib.dc_close(ctx)
 
     def find(info_string):