History test exists and passes.
History reimplemented; removed incorrect optimization.
Modified history_test to verify return value of publish.
Modified history to use lower consistency stores.
More detailed test for history.

We test multi-word structures and try to publish different values
from different threads. This should detect nonatomic behaviour of
reads and writes to/from buffer and latest.
Fix: Negative versions should be considered invalid.
Memory order changes in history.

The new version is more relaxed, but may compile to something worse on
x86 if misoptimized.
Added a locking implementation of history.
Holder implementations: both waitfree and locked.
Header guard added.