Add CXREF to changelog.dd.

The section on split needs to reference core.time, and changelog.dd does
not appear to use std.ddoc, so CXREF needs to be defined in changelog.dd
so that the links to core.time work.
Merge pull request #617 from jmdavis/split

Add CXREF to changelog.dd.
Announce beta 5
Struct constructors and static opCall cannot be mixed