- _execute_clauseelement() goes back to being

- _execute_clauseelement() goes back to being
a private method.  Subclassing Connection
is not needed now that ConnectionProxy
is available.
- tightened the interface for the various _execute_XXX()
methods to reduce ambiguity
- __distill_params() no longer creates artificial [{}] entry,
blank dict is no longer passed through to do_execute()
in any case unless explicitly sent from the outside
as in connection.execute("somestring"), {})
- fixed a few old sql.query tests which were doing that
- removed needless do_execute() from mysql dialect
- fixed charset param not properly being sent to
_compat_fetchone() in mysql