@sjaeckel integrated clang-format with formal coding style. Run his
script and commit changes.
There are pros and cons of this commit.
Mixed coding style is a "broken window". A good single style simplifies
reading and writing code.
On the other hand, this is a big change which will lead to conflicts.
C-style callbacks. Note that this requires the memory callbacks to
have a userdata pointer (so that we can pass the class instance). Also,
the Context class has to be passed into the constructors for Connection and
Stanza.
