Units: Improve arcsecond definitions

Improves on #876: Doesn't define `as` for `arcsecond`, only `mas` and `uas`.  Also makes up for the fact that we don't have a unit called `as` (which used to correspond `attosecond`) anymore, since it's a Python keyword and is quite confusing, though `attosecond` still exists for those crazy people that might need it.