Add user generic methods - #1849

import "pkl:ref"
before = ref.Reference(myDomain, MyReferent, myData)
after = ref.Reference::<MyDomain, MyReferent>(myDomain, myData)

// this can be cleaned up behind a generic typealias and method:
typealias MyRef<T> = ref.Reference<MyDomain, T>
function MyRef<T>(data: Any): MyRef<T> = ref.Reference::<MyDomain, T>(myDomain, data)
after2 = MyRef::<MyReferent>(myData)

// and references can now be constructed directly to complex types
complex1 = MyRef::<Listing<String>>(null)
complex2 = MyRef::<String | Int>(null)
complex3 = MyRef::<this>(null)
complex4 = MyRef::<(String) -> Int>(null)