Rework/enhance original htools opcodes

The original htools opcodes were minimalistic and not 1:1 equivalent
with the Python ones. Let's add all missing fields and, since we
changed the order, switch to more readable record syntax for building
the opcodes.

Signed-off-by: Iustin Pop <iustin@google.com>
Reviewed-by: Guido Trotter <ultrotter@google.com>