lib: split trace API into trace class and trace APIs

lib: split trace API into trace class and trace APIs

This patch adds the _trace class_ object and its API, moving some trace
API functions to this one instead.

The trace object was the only one holding both metadata and data objects
(stream classes and streams, for example). To be more consistent, a
trace class now deals with metadata only (no streams, no static state)
and a trace with data only. A trace is an instance (data streams) of a
trace class: you can have many traces described by the same trace class.

You create a trace class with no parameters, while you create a trace
with an existing trace class.

Like for the trace object, the trace class API has both const and
non-const APIs.

Both a trace and a trace class can have a name (optional), just like
both a stream and a stream class can have a name, and so on. With this
patch, I chose to set the name of the trace object in both src.ctf.fs
and src.text.dmesg, and to display the trace object's name in
sink.text.pretty.

Signed-off-by: Philippe Proulx <eeppeliteloop@gmail.com>