bug in function type parameter variance checking

[CeylonMigrationBot]

[@gavinking] The typechecker accepts this:

shared Boolean isInsideInterval<out Value>
        ([Value, Value] interval)(Value x)
        given Value satisfies Comparable<Value> 
        => let ([a,b]=interval) a <= x <= b;

That's inconsistent, since Value occurs contravariantly in the return type.

[Migrated from ceylon/ceylon-spec#1378]

[CeylonMigrationBot]

[@gavinking] This is pretty harmless and fixing it is too much of a PITA to bother with it for 1.2. I have added a comment indicating where the fix should go.