Added support for sequences to migrations.

Added the following migration operations: MoveSequence, RenameSequence, AlterSequence.
Specialized DatabaseBuilder and ModelDiffer for SqlServer because the annotations for sequences are only available there (should we move to Relational?)
In SqlServerModelDiffer the sequences are matched based on the matching columns.
Added unit tests.
Previously we only used 64-bit sequences and hence we always got back a long. Its good that we now support sequences of other sizes but raises some questions:
- Does Migrations use the size of the property to determine the size of the sequence to create?
- If so, what if multiple properties of different sizes share the same sequence?
- We already convert from long to the target type, if necessary, it seems we are now potentially converting from the target type to long and then back to the target type. This may not ultimately matter, but can it be avoided?