Added a new method :class:`.DefaultExecutionContext.current_parameters`
which is used within a function-based default value generator in
order to retrieve the current parameters being passed to the statement.
The new function differs from the ``.current_parameters`` attribute in
that it also provides for optional grouping of parameters that
correspond to a multi-valued "insert" construct. Previously it was not
possible to identify the subset of parameters that were relevant to
the function call.
Change-Id: I6894c7b4a2bce3e83c3ade8af0e5b2f8df37b785
Fixes: #4075
Mike Bayer
aed2324b4d