Changes to dealiasing and widening
Changes to dealiasing and widening

1) Mappers and Accumulators now reduce typerefs over refinements but do not follow general type args

Following type args in asSeenFrom leads into trouble because it specializes covariant aliases too early. A later as seen from might specialize the same alias with a more specific type.

2) The isDependent test in method type now uses a custom accumulator, because it needs to not flag erroenously types of the form p.T where p is a method paramater and T is a type parameter if `p`'s type which is aliased to an argument type. It should consider the argument type instead. With the change to 1), this broke, and is fixed by 2).

3) Unstable prefixes in member are widened.

VladimirNik/dotty,angi2/dotty,lampepfl/dotty,VladimirNik/dotty,som-snytt/dotty,lampepfl/dotty,DarkDimius/dotty,AlexSikia/dotty,reactormonk/dotty,folone/dotty,folone/dotty,AlexSikia/dotty,som-snytt/dotty,spetz911/dotty,dotty-staging/dotty,som-snytt/dotty,densh/dotty,dotty-staging/dotty,reactormonk/dotty,sjrd/dotty,sjrd/dotty,yusuke2255/dotty,angi2/dotty,lampepfl/dotty,densh/dotty,som-snytt/dotty,som-snytt/dotty,dotty-staging/dotty,spetz911/dotty,svalaskevicius/dotty,vsalvis/dotty,sjrd/dotty,auvik/dotty,sjrd/dotty,lampepfl/dotty,auvik/dotty,dotty-staging/dotty,lampepfl/dotty,dotty-staging/dotty,yusuke2255/dotty,svalaskevicius/dotty,sjrd/dotty,vsalvis/dotty