This introduces new methods on BaseEvolutionOperations for changing a

field type:


change_column_type(): Orchestrates changing the type of a field,

  dropping/re-adding any constraints and updating the types of any

  fields pointing to the changed field.


get_change_column_type_sql(): Returns backend-specific SQL for

  changing the type of any given field.


Along with a new capability attribute, change_column_type_sets_attrs,

which states whether get_change_column_type_sql() will also handle

setting all the attributes for the field definition. If False,

attributes need to be set in a separate change_column_attrs() call.
The basic strategy for changing a field type is:


Any constraints pointing to the changed field must be dropped, in

   order to avoid issues with incompatible data types between the

   referring fields and the changed field.


The field type can then safely be changed. Depending on the database

   backend, the command for this may include the entire field definition

   (NOT NULL, VARCHAR(...), etc.), or it may not (requiring that

   these changes are made separately, if needed). This part is not

   currently implemented by any backends.


Any fields referring to the changed field need to be updated to match

   the field type.


Constraints dropped in step 1 can then be restored.


BaseEvolutionOperations takes care of all of this by default, leaving

most backends responsible only for the capability attribute and the

implementation of get_change_column_type_sql(). Subclasses are free

to override the entire process, if needed.
Upcoming changes will implement the per-database support, the

orchestration in ChangeField and related objects, and the test suite.

Tested along with the other upcoming changes for this feature. All unit

tests pass on all supported databases and versions of Django.