Fix `Hash` and `Eq` for `ConstantOp` and `FillOp`

[mortendahl]

The way that Hash and Eq are currently implemented is not ideal, the former going through the (slower) textual representation and the latter not supposed to be implemented manually as it is now.

One root problem is around floating point values in constants, which are neither Hash nor Eq.

Maybe a solution would be to not have these values live directly on ConstantOp and FillOp, but rather keep a list of constants on the computation that ConstantOp and FillOp index into. Correctness would require this list of constants to be unique, so it's also worth considering other solutions that doesn't have this requirement (say, not using floating points in constants).