Minor fixes to resolve some compilation warnings

[LTLA]

This PR aims to eliminate a bunch of GCC/clang compiler warnings.

Most of these relate to unused variables. I've used the (void)variable pattern to trick the compiler into silencing the warning. I could have used [[maybe_unused]] but decided against it as that would force everyone to use C++17. Alternatively, I could have just removed the variable name from the function signature, but the names seemed to be informative (e.g., for the Base methods) so I left them in.

Other warnings were related to signed/unsigned comparisons when the template type S is an unsigned integer. While I was working on this, I ended up also fixing a small bug in the for loop for unsigned S. I also extended the test suite to compile and test for unsigned indices. In hindsight this might be better off as a separate PR.

There are still some remaining warnings in annoymodule.cc, but those are related to interactions with the Python C API and I'm uncertain on how to fix them. At least the warnings in annoylib.h have all been resolved.

Also FYI, ci.yml actually doesn't test anywhere other than Linux, {{ matrix.os }} isn't actually used anywhere. So we just end up running triplicate CI on Linux while pretending that it's running on Windows and Mac.