simplify internal storage of DML ordered values

simplify internal storage of DML ordered values

towards some refactorings I will need to do for #12496, this
factors out the "_ordered_values" list of tuples that was used to
track UPDATE VALUES in a specific order.   The rationale for this
separate collection was due to Python dictionaries not maintaining
insert order.   Now that this is standard behavior in Python 3
we can use the same `statement._values` for param-ordered and
table-column-ordered UPDATE rendering.

Change-Id: Id6024ab06e5e3ba427174e7ba3630ff83d81f603