Add bt_stream_pos to bt_trace_descriptor

This is intended to provide a generic way to access the bt_stream_pos API
without having to make assumptions about the output format's descriptor layout.
Fix code relying on the output format being ctf-text
Add a format-agnostic iterator creation mechanism

Iterators now expose an event providing function so that a user does not
have to know the input trace format to get "bt_ctf_event"s.

A context can now create a specialized iterator depending on its input
trace format.