This introduces BaseEvolutionOperations.build_column_schema(), which

will compute pieces of SQL needed in order to create or re-create a

column.
This sort of operation has been implemented in a few different spots in

the codebase, in a few different ways. This consolidates all these,

simplifying the code paths, easing maintainability, and setting things

up for an upcoming change for field type changes.
There is one change to the generation that impacts the SQLite3 tests,

but not any actual operation. The old logic would generate a UNIQUE

for a PRIMARY KEY, which wasn't at all necessary. The new logic avoids

that.

Unit tests pass for all versions of Django and all databases.