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>

This commit adds the design document for introducing "reason trails",
tracing the reason why opcodes are executed, step by step.

Signed-off-by: Michele Tartara <mtartara@google.com>
Reviewed-by: Guido Trotter <ultrotter@google.com>