Subtyping for Explicit Casts

[DavePearce]

(see also #845)

At the moment, there is no sutyping operator for casts. Thus, the following fails:

type Link is {int data}
type BigLink is {int data, int code}

function sum(Link l) -> int:
    return l.data

function sum2(BigLink l) -> int:
  return sum((Link) l)

[DavePearce]

This is also causing me problems moving between different instances of dom::Element