More specifics on the implementation of the reason trail

Adding the reason trail as a parameter instead of as a new class attribute fits
better with the current structure of the system.

Also, the description of what part of the implementation will be OpCode
specific and which will be shared is added.

Signed-off-by: Michele Tartara <mtartara@google.com>
Reviewed-by: Helga Velroyen <helgav@google.com>