Rewrites don't preserve global phase

[ss2165]

There is no phase tracking, and no phase gate in the standard set.
It is not a natural fit because it is a non-dataflow global operation. Almost like metadata?

Phase only matters in certain kinds of circuit composition (e.g. quantum control).

[aborgna-q]

We should be keeping the pytket global phase on the circuit metadata. Do you have a failing example?
https://github.com/CQCL/tket2/blob/main/tket2%2Fsrc%2Fserialize%2Fpytket%2Fencoder.rs#L57-L62

(There's currently no way for rewrites to modify the phase)