CTF writer: use own `bt_ctf_object` and `bt_ctf_value` internal APIs

CTF writer: use own `bt_ctf_object` and `bt_ctf_value` internal APIs

Removing CTF writer's implementation's dependency on `bt_object` and
`bt_value` makes it even more independent so as to be free to change the
`bt_object` and `bt_value` APIs without having to update the CTF writer
part.

Both APIs are not public as of this patch, so I'm hiding the public CTF
writer functions to get a trace's environment field (and their tests),
which is a `bt_ctf_value` object. This is not breaking backward
compatibility with Babeltrace 1.5.

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