ust registry: Refactor representation of nested types

ust registry: Refactor representation of nested types

Those allow LTTng-UST to internally represent nested types properly,
and to serialize them when sending them over to the session
daemon. However, for now, the session daemon only accepts arrays and
sequences which contain an integer type, which is the only use-case
emitted by lttng-ust. Wait until we have the nested types wired up
within lttng-ust before accepting and supporting nested types so it
can be tested.

Given the size of this change to ust-metadata.c, use this opportunity
to make sure that every

  if (ret)
      ...

in this file is changed to conform to lttng-tools coding style:

  if (ret) {
      ...
  }

Signed-off-by: Mathieu Desnoyers <mathieu.desnoyers@efficios.com>
Signed-off-by: Jérémie Galarneau <jeremie.galarneau@efficios.com>
Change-Id: I3ac44fc3a993f73d1fb08056331ad6fed7059981
Depends-on: lttng-ust: I45bb0886c5ac970c3ca75dbefcb94adb50801294