Fix compiler warnings

[MichaelBroughton]

When compiling our C++ code we are starting to get a lot of (mostly) harmless warnings about comparison between unsigned and signed types etc. It would be nice if we could comb through these and make the tweaks to make them go away. This isn't an urgent issue, but it would be nice to clean up the clutter.

[mhucka]

This has been pretty much completed via these PRs:

• Fix type mismatch warnings #839
• Fix warnings about unused variables in circuit_parser_qsim.cc #838
• Refactor to move BalanceTrajectory to separate file #837
• Fix "unused lambda capture" compiler warnings #836
• Add bazel flags to silence compiler warnings we can't do anything about #835
• Replace deprecated INSTANTIATE_TEST_CASE_P #834