sessiond: express field references as locations instead of names

sessiond: express field references as locations instead of names

CTF2 requires type references (a sequence's length, a variant's
selector) to be expressed as absolute field locations.

The `field` API currently expresses these references as names directly
since those are provided by the tracer.

This change introduces the notion of a field location and adapts
the tsdl visitor to serialize those as expected.

Signed-off-by: Jérémie Galarneau <jeremie.galarneau@efficios.com>
Change-Id: I56f603062d6748051adf3fa31bc08422c47c144d