Let check always succeed on 'id' type

As is the Objective-C way.
Cocoa collection classes are arbitrary KVC containers
Allow class annotation to denote arbitrary KVC container

    __attribute__((annotate("objc_kvc_container")))
    @inteface MyContainer : NSObject
    @end

New top-level attributes (i.e. `__attribute__((objc_kvc_container))`)
can't be added by plug-ins, so use annotate.
valueForKey `foo` can be satisfied by `isFoo`