Assort IWYU Cleanups
include-what-you-use is a relatively common tool for cleaning up includes. it helps to localize includes and identify unnecessary ones

It has a few rough edges that don't make it 100% compatible with our code base, so I'm not sure we can do this in CI. But here were a few cleanups I manually gleaned from the IWYU output that I think make sense
Thanks @WillAyd 