Replace Builder's From type parameter with a Scope type member

Unlike with the From type parameter, Builder's implicit scope does not
include the new Scope type member (SLS §7.2). This means that implicit
Builder resolution will no longer search the source type's companion
object. But since collections have a relatively shallow type
hierarchy–and the interfaces have no companion objects anyway–this
should not pose a problem. Builder finally looks like it should!

Also resolve Buffer/Builder specialization conflicts, enabling removal
of Accumulator. Buffer now extends Builder, but leaves its State type
member abstract.