Eliminate nqp::p6decont for plain nqp::decont.

We'll attach the Rakudo semantics/optimization directly to the type
using the new container spec handling.
Update nqp::setcontspec usages for new protocol.
Adopt the new container spec model.

This doesn't go quite so far as the last attempt to do this. It does
enough to get the cleaner/more extensible API and a small type safety
(and perhaps minor performance) win, plus will be easier to implement
on the JVM.