Changelog:
============
- Added Operations.implementation_for.replace parameter to Operations.
implementation_for(), allowing replacement of existing operation implementations.
- Fixed issue in SQL Server dialect where the DROP that's automatically emitted
for existing default constraints during an ALTER COLUMN needs to take place
before not just the modification of the column's default, but also before the column's type is changed
Signed-off-by: Wang Mingyu <wangmy@fujitsu.com>
Signed-off-by: Khem Raj <raj.khem@gmail.com>
