Reduce import time overhead

Reduce import time overhead

* Fix subclass traversals to not run classes multiple times

* switch compiler visitor to use an attrgetter, to avoid
  an eval() at startup time

* don't pre-generate traversal functions, there's lots of these
  which are expensive to generate at once and most applications
  won't use them all; have it generate them on first use instead

* Some ideas about removing asyncio imports, they don't seem to
  be too signficant, apply some more simplicity to the overall
  "greenlet fallback" situation

Fixes: #5681
Change-Id: Ib564ddaddb374787ce3e11ff48026e99ed570933