Add CircuitFunction

[arettig]

This adds CircuitFunction, which maps from a set of parameters to a
circuit.CircuitFunction will be used by the planned CallFunction
operation in a future PR. This is an experimental Cirq 2.0 feature.

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 99.59%. Comparing base (4198478) to head (e2e2e8f).
⚠️ Report is 1 commits behind head on main.

Additional details and impacted files

@@           Coverage Diff            @@
##             main    #8314    +/-   ##
========================================
  Coverage   99.59%   99.59%            
========================================
  Files        1125     1127     +2     
  Lines      103250   103421   +171     
========================================
+ Hits       102828   103000   +172     
+ Misses        422      421     -1     

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