Avoid Moment typechecks when (un)freezing circuits

[maffoo]

When freezing or unfreezing circuits, we have a list or tuple of Moment objects so we can call the _from_moments classmethod on the relevant target type which avoids the isinstance typechecks that are needed when calling from_moments as we do currently.

See below for a microbenchmark of this change. On main, freezing an example circuit with 10_001 moments takes something like 550us, while on this branch the same freeze operation takes about 40us (note that we have to copy the circuit before freezing because we cache the result of freeze if a circuit has not been mutated).

With main:

In [1]: import cirq; q = cirq.q(0); c = cirq.Circuit([cirq.X(q)] * 10_000, cirq.M(q))

In [2]: %timeit c.copy()
25.6 μs ± 135 ns per loop (mean ± std. dev. of 7 runs, 10,000 loops each)

In [3]: %timeit c.copy().freeze()
575 μs ± 821 ns per loop (mean ± std. dev. of 7 runs, 1,000 loops each)

With this branch:

In [1]: import cirq; q = cirq.q(0); c = cirq.Circuit([cirq.X(q)] * 10_000, cirq.M(q))

In [2]: %timeit c.copy()
26 μs ± 137 ns per loop (mean ± std. dev. of 7 runs, 10,000 loops each)

In [3]: %timeit c.copy().freeze()
66.2 μs ± 1.78 μs per loop (mean ± std. dev. of 7 runs, 10,000 loops each)

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 98.13%. Comparing base (ce31720) to head (b8e2378).
Report is 1 commits behind head on main.

Additional details and impacted files

@@           Coverage Diff           @@
##             main    #7150   +/-   ##
=======================================
  Coverage   98.13%   98.13%           
=======================================
  Files        1093     1093           
  Lines       95579    95579           
=======================================
+ Hits        93795    93797    +2     
+ Misses       1784     1782    -2     

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[pavoljuhas]

Thank you!