to start up listener collections; this pulls the overhead off of construction
and makes performance much like the descriptor version, while still allowing
slots.   Fix up some profiles.

allowing us to move to __slots__

size of the many per-column objects we're hitting, but somehow the overall memory is
hardly being reduced at all in initial testing

sqlalchemy/orm, sqlalchemy/event, sqlalchemy/testing

to get all flake8 passing

Removed ungrammatical apostrophes from documentation, replacing
"it's" with "its" where appropriate (but in a few cases with "it is"
when that read better).

While doing that, I also fixed a couple of minor typos etc.
as I noticed them.