lib: add BT_LIB_LOG*() macros to log/serialize library objects

Add specialized, internal logging macros to log Babeltrace library
objects. Those macros expand to calling bt_lib_log(), which calls
bt_common_custom_vsnprintf() to support custom conversion specifiers.

Almost all Babeltrace objects are supported. The CTF writer object is
supported with the `%!w` specifier, but its formatting remains to be
done. There is no conversion specifier to format a clock class priority
map.

See `include/babeltrace/lib-logging-internal.h` for more details,
limitations, and for the list of custom conversion specifiers.

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