Apply variable-spaced optimization to QROM circuits

[tanujkhattar]

Updates the unary iteration framework to allow "early termination" and uses this trick to apply the variable-spacing optimization to QROM circuits described in https://arxiv.org/pdf/2007.07391.pdf

As a drive-by fix, the PR also fixes a flaky bug in GreedyQubitManager where the free'd qubits were added to the pool of managed qubits in a non-deterministic order due to finding unique elements via set(qubits). Replaced with an order-preserving way of finding unique elements.

[fdmalone]

LGTM % adding an additional test case.

[tanujkhattar]

@fdmalone Added more tests and also updated the _value_equality_values_ method because the T-complexity will now depend upon the data to be loaded and not just on the shape of the registers.

[fdmalone]

LGTM

[codecov]

Codecov Report

Patch coverage: 100.00% and project coverage change: -0.01% ⚠️

Comparison is base (6e38a27) 97.60% compared to head (384bc61) 97.60%.
Report is 3 commits behind head on master.

Additional details and impacted files

@@            Coverage Diff             @@
##           master    #6257      +/-   ##
==========================================
- Coverage   97.60%   97.60%   -0.01%     
==========================================
  Files        1116     1116              
  Lines       95772    95815      +43     
==========================================
+ Hits        93481    93519      +38     
- Misses       2291     2296       +5     

Files Changed	Coverage Δ
cirq-core/cirq/protocols/json_serialization.py	97.68% <ø> (ø)
cirq-ionq/cirq_ionq/ionq_devices_test.py	100.00% <ø> (ø)
cirq-core/cirq/vis/heatmap_test.py	98.00% <100.00%> (-0.02%)	⬇️
cirq-ft/cirq_ft/algos/qrom.py	100.00% <100.00%> (ø)
cirq-ft/cirq_ft/algos/qrom_test.py	100.00% <100.00%> (ø)
cirq-ft/cirq_ft/algos/unary_iteration_gate.py	96.26% <100.00%> (+0.14%)	⬆️
cirq-ft/cirq_ft/infra/qubit_manager.py	100.00% <100.00%> (ø)
cirq-ft/cirq_ft/infra/qubit_manager_test.py	100.00% <100.00%> (ø)
cirq-ionq/cirq_ionq/ionq_client.py	100.00% <100.00%> (ø)
cirq-ionq/cirq_ionq/ionq_client_test.py	100.00% <100.00%> (ø)

... and 2 files with indirect coverage changes

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