Encapsulate check logic into Attr.ResultInfo

The check routine in Check.checkType is very specific to assignment/return contexts. As it is, that routine cannot be re-used in different contexts (most notably, method check contexts).

This is problematic when moving the code forward - in the case of lambda expressions, a lambda can be type-checked in the middle of overload resolution. We need a slick way to define the type-checking routine in a transparent way w.r.t. check context.