Fix: Suppress a compiler warning (always-false condition)

Clang reports an always-false condition (checking if an unsigned value < 0)
caused by a type mismatch in ctf-visitor-generate-io-struct.c.
Remove unused array in bt_context_add_traces_recursive
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.