[WIP] Add support for comparing DEMs up to instruction ordering

[abbrazi]

The current == overload of the DetectorErrorModel only returns true if the DEMs have the same instruction ordering. It is sometimes useful to be able to compare two DEMs for equality when this is not the case, so I implemented an equal_up_to_instruction_ordering member function. I opted not to change the == operator because (a) checking for equality up to instruction ordering requires sorting so turns an O(n) op into an O(n log n) op; and (b) it's possible some of the backend simulation code depends on the ordering constraint.

[Strilanc]

Two major problems with this method as currently implemented.

1. Instruction order sometimes matters. In particular, shift_detector instructions change the meaning of later error instructions. If you simply sort the instructions without account for this, you will lose this dependence and break the dem.

2. Not efficient when loops are present. It seems to me that the intent of the method is to basically be graph isomorphism, but that would imply these three dems should all be equal:

error(0.1) D0 D1
shift_detectors 1
error(0.1) D0 D1
shift_detectors 1
error(0.1) D0 D1

and

error(0.1) D0 D1
repeat 2 {
    shift_detectors 1
    error(0.1) D0 D1
}

and

repeat 2 {
    error(0.1) D0 D1
    shift_detectors 1
}
error(0.1) D0 D1

a simple solution would be to first flatten the dem, but that would make the method take far too long when applied to circuits with high repetition counts. There are a few exceptions, but basically all methods in stim are efficient when applied to high-loop-count circuits. This method would also need to meet that bar.

[abbrazi]

Yeah I guess this is a flat world theory :D
It's more interesting than I thought, let me think about it.