Extract int and char arrays from fields

Three helper functions to ease the extraction of basic types from event
fields. For now we support signed and unsigned integers and strings.
Other functions could be added following the same principle for other
types.

Signed-off-by: Julien Desfossez <julien.desfossez@polymtl.ca>
Signed-off-by: Mathieu Desnoyers <mathieu.desnoyers@efficios.com>