Future migration warning for with type operator

[nicolasstucki]

• In future-migration we emit the deprecation warning and enable the patch with -rewrite.
• In future we emit we make this syntax an error

//> using options  -source future
def foo: Int with String = ??? // error

//> using options  -rewrite -source future-migration
- def foo: Int with String = ???
+ def foo: Int & String = ???