mean_estimation.complex_phase_oracle_test.test_phase_oracle is very slow because of Fxp

[mpharrigan]

This is the change: https://github.com/quantumlib/Qualtran/pull/1041/files#r1638968757

Somehow, this has turned the parameterized unit test from a 1 second to a 21 second ordeal on my machine.

Is it worth resuscitating our own fixed-point-bits function? or is there a better incantation to the Fxp library to get it to go faster?

Perhaps related: https://github.com/quantumlib/Qualtran/issues/643#issuecomment-2065491533