Add PrimitiveExt trait

[fintelia]

Users cannot currently write their own generic methods that call methods bounded by the (not publicly exported) Enlargeable trait. Rather than making that trait public and thus exposing its methods and associated type to users as #2451 proposed, this PR experiments with adding a new sealed trait PrimitiveExt that extends Enlargeable but has no methods of its own.

This is a intended as stopgap measure since we cannot make Primitive a sealed trait until doing another breaking release.