ctf-writer: externalize libbabeltrace2-ctf-writer

Completely split the ctf writer part from the main babeltrace2 library.

 * Remove the obsolete libbabeltrace2-ctf library
 * Add a new libbabeltrace2-ctf-writer library
 * Remove the ctf writer includes from the main include file
 * Fork an internal copy of assert-pre.h that doesn't use the library
   structures.
 * Add a logging context specific to ctf writer

Signed-off-by: Michael Jeanson <mjeanson@efficios.com>
Change-Id: Iff9ab62ee9eeb8abd290fb1d758a73050e21c99b
Reviewed-on: https://review.lttng.org/c/babeltrace/+/1410
Tested-by: jenkins
Reviewed-by: Philippe Proulx <eeppeliteloop@gmail.com>