Remove "subtyping" based variants

[kritzcreek]

The idea was cute:

variant V {
  struct A { a : Int }
}

match v {
  V::A a => a // has type `V::A` here, which is `struct { a : int }`
}

but to make it work properly I need full subtyping, and I'd rather not. This means switching to pattern matching that has to extract the fields and bind them to local variables.