getting them all compatible with py.test as well

the first SQL expression would be applied as the "comparison type"
to a compared tuple value; this has the effect in some cases of an
inappropriate "type coersion" occurring, such as when a tuple that
has a mix of String and Binary values improperly coerces target
values to Binary even though that's not what they are on the left
side.  :func:`.tuple_` now expects heterogeneous types within its
list of values.
fixes #2977

isn't appended to the list.  reflection makes use of this.

a no-name :class:`.BindParameter` is received, e.g. via :func:`.sql.literal`
or similar; the "key" of the bind param is used as the key within
.c. rather than the rendered name.  Since these binds have "anonymous"
names in any case, this allows individual bound parameters to
have their own name within a selectable if they are otherwise unlabeled.
fixes #2974

when presented with duplicate columns.  The behavior of emitting a
warning and replacing the old column with the same name still
remains to some degree; the replacement in particular is to maintain
backwards compatibility.  However, the replaced column still remains
associated with the ``c`` collection now in a collection ``._all_columns``,
which is used by constructs such as aliases and unions, to deal with
the set of columns in ``c`` more towards what is actually in the
list of columns rather than the unique set of key names.  This helps
with situations where SELECT statements with same-named columns
are used in unions and such, so that the union can match the columns
up positionally and also there's some chance of :meth:`.FromClause.corresponding_column`
still being usable here (it can now return a column that is only
in selectable.c._all_columns and not otherwise named).
The new collection is underscored as we still need to decide where this
list might end up.   Theoretically it
would become the result of iter(selectable.c), however this would mean
that the length of the iteration would no longer match the length of
keys(), and that behavior needs to be checked out.
fixes #2974
- add a bunch more tests for ColumnCollection

of columns given positionally would not be preserved.   This could
have potential impact in positional situations such as applying the
resulting :class:`.TextAsFrom` object to a union.

constructs has been enhanced in order to assist with existing
schemes that rely upon addition of ad-hoc keyword arguments to
constructs.
- To suit the use case of allowing custom arguments at construction time,
the :meth:`.DialectKWArgs.argument_for` method now allows this registration.
fixes #2962