Typechecker isSubtype does not handle alias type

[Mark1626]

https://github.com/cucapra/dahlia/blob/master/src/main/scala/typechecker/Subtyping.scala#L44-L48

The following results in an error from the type checker

Case 1

record point { x: bit<32> }
def f(p: point): point = {
    let nextp: point = { x=p.x + 1 };
    return nextp;
}

Expected subtype of point in return, received: point

In this case areEqual receives the variable np as type TRecType while the return type is considered as TAlias.

---

Case 2

record point { x: ubit<32> }
let a: point = {x=10};
let b: point = (a as point);

The opposite scenario, areEqual receives TAlias for t1 and TRecType for t2.