Deprecate tuple syntax for object creation

[nrktkt]

the current tuple syntax

obj("field" -> value)

gives less useful compiler messages than the long or squiggly arrow syntax

obj("field" ~> value, "field2" --> value2)

We should deprecate the magnet based tuple syntax. It could be available as an import if users really want it, but it shouldn't be the default that new users are exposed to.