Cleans up all warnings seen w/GCC 9.1 to allow it to track the main
branch more easily until 3.x.
Does not include Ticker.h "fix" of pragmas around a function cast we're
doing that GCC9 doesn't like, that will be addressed separately and
maybe only in the 3.0 branch.
Does not include GDB hook fix, either, because the pragmas required
to disable the GCC9.1 warnings don't exist in 4.8 at all.
52f8c62b81