ctfser: have dedicated signed/unsigned integer writing functions

Most of the time there's an external check for signedness anyway, so it
is redundant to pass the signedness to the writing function only for it
to evaluate it again.

In `lib/ctf-writer/fields.c`, the signedness check is now peripheral to
the ctfser writing function.

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