Fix: access field properties by declaration

The functions that give access to the properties of the fields (int
signedness, base, char len, etc), were only working with definition
struct which prevented their use before actually reading the events.

This patch changes the prototypes of these functions to use
declarations instead of definitions and provides a converter
function to access the declaration from a definition.

Signed-off-by: Julien Desfossez <jdesfossez@efficios.com>
Signed-off-by: Mathieu Desnoyers <mathieu.desnoyers@efficios.com>