Fix problem in unapply typing.

GADT bound resetting may only be applied when comparing
<pattern type> <: <expected type>, not when comparing the
other way around.

The fix revealed an error in a test case (t1048) which looks like a real error.
Therefore the test got moved to neg.
Fix rewriting of by-name class parameter accessors.

Like the corresponding parameters, these also need to be rewritten to function types.
Completed constructors phase

1) Type parameters are now copied to accessors
2) Constructors also work for traits

2) makes it possible do to mixin after constructors.