Piccolo

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Changing the status code when creating a migration, and no changes were
detected. It now returns a status code of 0, so it doesn't fail build scripts.

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Added ``Bytea`` / ``Blob`` column type.

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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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