Piccolo

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Fixing a bug with migrations which drop column defaults.

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* Fixing a bug where re-running ``Table.create(if_not_exists=True)`` would
fail if it contained columns with indexes.
* Raising a ``ValueError`` if a relative path is provided to ``ForeignKey``
``references``. For example, ``.tables.Manager``. The paths must be absolute
for now.

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Fixing a bug with ``Boolean`` column defaults, caused by the ``Table``
metaclass not being explicit enough when checking falsy values.

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* The ``ForeignKey`` ``references`` argument can now be specified using a
string, or a ``LazyTableReference`` instance, rather than just a ``Table``
subclass. This allows a ``Table`` to be specified which is in a Piccolo app,
or Python module. The ``Table`` is only loaded after imports have completed,
which prevents circular import issues.
* Faster column copying, which is important when specifying joins, e.g.
``await Band.select(Band.manager.name).run()``.
* Fixed a bug with migrations and foreign key contraints.

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Modified the exit codes for the ``forwards`` and ``backwards`` commands when no
migrations are left to run / reverse. Otherwise build scripts may fail.

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* Improved the method signature of the ``output`` query clause (explicitly
added args, instead of using ``**kwargs``).
* Fixed a bug where ``output(as_list=True)`` would fail if no rows were found.
* Made ``piccolo migrations forwards`` command output more legible.
* Improved renamed table detection in migrations.
* Added the ``piccolo migrations clean`` command for removing orphaned rows
from the migrations table.
* Fixed a bug where ``get_migration_managers`` wasn't inclusive.
* Raising a ``ValueError`` if ``is_in`` or ``not_in`` query clauses are passed
an empty list.
* Changed the migration commands to be top level async.
* Combined ``print`` and ``sys.exit`` statements.

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