Check GADT ordering for HKTs in TypeComparer

[Linyxus]

Since the current TypeComparer calls GadtConstraint.bounds to check GADT bounds of HKTs, the GADT ordering between two HK type parameters will not be available when comparing them. The following snippet illustrates the issue.

final class HktInv[F[_]]
final class HktVar[+F[_]]

def foo[F[_], G[_], X](m : HktInv[G]) = m match {
  case _ : HktInv[F] =>
    val fx : F[X] = ???
    val gx : G[X] = fx  // pass! `F` has been instantiated to `G` 
                        // and thus the bound F =:= G is accessible from `bounds`.
}

def baz[F[_], G[_], X](m : HktVar[G]) = m match {
  case _ : HktVar[F] =>
    val fx : F[X] = ???
    val gx : G[X] = fx  // error. `F` has only ordering constraints F <:< G 
                        // and such ordering will not be available to TypeComparer via `bounds`.
}

The orderings will be available via fullBounds, but this method may result in dead loops if used in TypeComparer. This PR fixes this issue by checking ordering constraints between HKTs with isLess in TypeComparer.

@abgruszecki

[abgruszecki]

LGTM! Good job with the fix.