Add stim.Flow and stim.Circuit.has_all_flows (#700)

- Add `stim.Flow` class for representing stabilizer flows
- Refactor `stim.has_flow` to only take a `stim.Flow` [breaking change
vs dev versions]
- Add `stim.Circuit.has_all_flows` method for more efficiently checking
flows

In C++ land:
- Refactor more methods to take `std::string_view` instead of `const
char *` or `std::string`
- Rename `stim::StabilizerFlow<W>` -> `stim::Flow<W>`
- Refactor `stim::Flow<W>` to allow non-negative measurement indices

file_lists/python_api_files

@@ -20,6 +20,7 @@ src/stim/simulators/frame_simulator.pybind.cc
 src/stim/simulators/matched_error.pybind.cc
 src/stim/simulators/measurements_to_detection_events.pybind.cc
 src/stim/simulators/tableau_simulator.pybind.cc
+src/stim/stabilizers/flow.pybind.cc
 src/stim/stabilizers/pauli_string.pybind.cc
 src/stim/stabilizers/pauli_string_iter.pybind.cc
 src/stim/stabilizers/tableau.pybind.cc

file_lists/test_files

@@ -4,7 +4,6 @@ src/stim/circuit/circuit.test.cc
 src/stim/circuit/export_qasm.test.cc
 src/stim/circuit/gate_decomposition.test.cc
 src/stim/circuit/gate_target.test.cc
-src/stim/circuit/stabilizer_flow.test.cc
 src/stim/cmd/command_analyze_errors.test.cc
 src/stim/cmd/command_convert.test.cc
 src/stim/cmd/command_detect.test.cc
@@ -72,6 +71,7 @@ src/stim/simulators/transform_without_feedback.test.cc
 src/stim/simulators/vector_simulator.test.cc
 src/stim/stabilizers/conversions.test.cc
 src/stim/stabilizers/flex_pauli_string.test.cc
+src/stim/stabilizers/flow.test.cc
 src/stim/stabilizers/pauli_string.test.cc
 src/stim/stabilizers/pauli_string_iter.test.cc
 src/stim/stabilizers/pauli_string_ref.test.cc