Add Trace CTF IR type

Refactored the Writer interface to expose a Trace object which
will be exposed by the CTF reader.

Signed-off-by: Jérémie Galarneau <jeremie.galarneau@efficios.com>
Add internal nameless clock creation API

Signed-off-by: Jérémie Galarneau <jeremie.galarneau@efficios.com>
Add an alias name attribute and accessor to CTF IR field type

Signed-off-by: Jérémie Galarneau <jeremie.galarneau@efficios.com>