Add caching to value_equality_values decorator for auto generated methods.

[tanujkhattar]

Fixes #6272
cc #6097

Benchmarks can be seen by running

>>> gate = cirq.PhasedXZGate.from_matrix(cirq.testing.random_unitary(2))
>>> %timeit hash(gate)

Before

2.67 µs ± 52.8 ns per loop (mean ± std. dev. of 7 runs, 100000 loops each)

After

159 ns ± 2.75 ns per loop (mean ± std. dev. of 7 runs, 10000000 loops each)

A more real world example would be QROM in Cirq-FT, where value_equality_values returns all the data to be loaded by the QROM. Benchmarks as follows

>>> qrom = cirq_ft.QROM.build([1, 2, 3, 4, 5] * 10_000)
>>> %timeit cirq_ft.t_complexity(qrom)

Before

# 3.39 ms ± 32.2 µs per loop (mean ± std. dev. of 7 runs, 1 loop each)

After

# First Run
# The slowest run took 14.37 times longer than the fastest. This could mean that an intermediate result is being cached.
# 6.05 µs ± 9.3 µs per loop (mean ± std. dev. of 7 runs, 1 loop each)

# Second Run
# 1.85 µs ± 37.4 ns per loop (mean ± std. dev. of 7 runs, 1000000 loops each)

[pavoljuhas]

Nit: _compat.cached_method(values_getter) can be reused so that the values_getter is evaluated once. Otherwise LGTM.

[codecov]

Codecov Report

Patch coverage: 100.00% and no project coverage change.

Comparison is base (0e80fa5) 97.87% compared to head (3326064) 97.88%.

Additional details and impacted files

@@           Coverage Diff           @@
##           master    #6275   +/-   ##
=======================================
  Coverage   97.87%   97.88%           
=======================================
  Files        1106     1106           
  Lines       95710    95717    +7     
=======================================
+ Hits        93679    93690   +11     
+ Misses       2031     2027    -4     

Files Changed	Coverage Δ
cirq-core/cirq/ops/dense_pauli_string.py	97.97% <100.00%> (+0.01%)	⬆️
cirq-core/cirq/ops/linear_combinations.py	99.79% <100.00%> (ø)
cirq-core/cirq/value/value_equality_attr.py	96.87% <100.00%> (+0.26%)	⬆️

... and 1 file with indirect coverage changes

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