Take the table name from the schema instead of the operation, to account
for any renames that may have occurred by preceding operations in the
  same migration

Add a `Deleted` field to the `Column` struct in order to allow
`OpDropColumn` to soft delete columns in the virtual schema.

Update `RemoveColumn` to set the `Deleted` field to `true` instead of
removing a column from the `columns` map.

Add an `UnRemoveColumn` method to allow columns to be unremoved. This can
be used by `OpDropColumn` to rollback the effects of a column deletion
in the virtual schema.

When looking up a column in the schema, ignore columns that have been
marked as deleted.

When creating views in the new schema, skip columns that have been marked
as deleted.

Take the name of the column whose addition should be rolled back by
looking up the name of the column in the schema, rather than assuming
the name is the one given in the operation; this ensures that
indirections are handled correctly.

Mark the dropped column as no longer deleted so that it becomes visible
to preceding rollback operations in the same migration.