Improve warnings

[BrendanKKrueger]

PR Summary

• Eliminate some existing warnings
  • Some variables in the tests are only used for compile-time checks, and thus trip over unused-variable warnings. Decorate those with [[maybe_unused]].
  • Eliminated warnings about calling __host__-only functions from __host__ __device__ functions by replacing std::vector in some tests with a very simple non-trivial type.
  • Modify array::swap to be device-capable (std::swap is not constexpr until C++20).
  • Because the actual type used for std::size_t is platform and compiler dependent, @mauneyc-LANL would sometimes see warnings about comparing unsigned integers against zero. Replaced the checks that I could find with a small function that won't trip the warnings.
• Copied the warning flags from Nautilus: adds more warnings, promotes warnings to errors.
• Miscellaneous small changes for the new warning flags.

PR Checklist

• Any changes to code are appropriately documented.
• Code is formatted.
• Install test passes.
• Docs build.
• If preparing for a new release, update the version in cmake.

[Yurlungur]

Thanks for this cleanup! As soon as tests pass, I'm happy. I think the current tests issue is that span_EXPECTS needs to typedef'd to be defined explicitly to a no-op for non-debug builds to avoid this warning (now promoted to error):

#define span_EXPECTS(...) assert((__VA_ARGS__))