ChangeField now takes a field_type= option, which is used to change

the type of the field. If the database-specific types differ, then this

will go through the new BaseEvolutionOperations.change_column_type()

machinery to change the type of the field and, depending on the

database, any fields referencing it.
This is also supported when generating hinted evolutions. If the differ

sees that the type of a field has changed, it will generate a sample

ChangeField that changes the type and passes in all attributes (not

just those that would otherwise have changed) so that the backend can

generate a complete column definition.
Changes to fields that don't end up actually altering the type (such as

when two fields map to the same type at the database level) are treated

as a standard ChangeField operation, modifying any changed attributes.
Unit tests have been added for all database backends and for several

field type change scenarios.

All unit tests pass on all supported versions of Python, all versions of

Django, and various versions of all supported database backends.