Rename CTF IR integer FT signedness functions to use "is"

Rename CTF IR integer FT signedness functions to use "is"

bt_ctf_field_type_integer_get_signed() becomes
bt_ctf_field_type_integer_is_signed().

bt_ctf_field_type_integer_set_signed() becomes
bt_ctf_field_type_integer_set_is_signed().

Backward compatibility with pre-2.0 CTF writer is kept with the help
of CPP definitions.

Signed-off-by: Philippe Proulx <eeppeliteloop@gmail.com>
Signed-off-by: Jérémie Galarneau <jeremie.galarneau@efficios.com>