Revert two commits related to Duration

Serious conceptual problems here, see
http://irclog.perlgeek.de/perl6/2013-12-01#i_7938521
for discussion.

This reverts commit 1735a4652b3885a13ce14c654af2a5b90c915bd0.
This reverts commit c32379ef139b153bc380557935f87b09310cb98a.