Fix: move "user" attribute from field to type

The "user" field attribute (copy from userspace) is not taken into
account in the bytecode specialization and interpreter recursive
traversal through composed types (LOAD_FIELD bytecode instructions).

Those are currently used by the filter expressions which access fields
nested within composed types.

Move the "user" attribute from the event fields to the integer and
string types. This will allow ensuring that the bytecode specialization
and interpreter have access to this user attribute even in nested types
(e.g. arrays, sequences) in a subsequent change.

Signed-off-by: Mathieu Desnoyers <mathieu.desnoyers@efficios.com>
Change-Id: I093d61c98bf725b2974f4a19cae560b7fc147f4f