- added an "inline_ddl" flag to Constraint. this controls if DDL is emitted

- added an "inline_ddl" flag to Constraint.  this controls if DDL is emitted
from within CREATE TABLE, and is flipped off automatically when the item
is placed within an Add/DropConstraint object.
- the use_alter flag on ForeignKey is now a shortcut option for operations that
can be hand-constructed using the DDL() event system.  A side effect of this refactor
is that ForeignKeyConstraint objects with use_alter=True will *not* be emitted on
SQLite, which does not support ALTER for foreign keys.  This has no effect on SQLite's
behavior since SQLite does not actually honor FOREIGN KEY constraints.
- the "on" callable passed to DDL() needs to accept **kw arguments.
In the case of MetaData before/after create/drop, the list of
Table objects for which CREATE/DROP DDL is to be issued is passed
as the kw argument "tables".   This is necessary for metadata-level
DDL that is dependent on the presence of specific tables.
- the "metadata" argument is removed from DefaultGenerator and subclasses,
but remains locally present on Sequence, which is a standalone construct
in DDL.
- really trying to pare down usage of testing.mock_engine()