Refactor allocation policies.

Refactor allocation policies.

Converted allocation policy classes from static namespaces to stateful
objects. FooAllocationPolicy renamed to FooAllocator.

This allowed zone allocation policy to have a direct pointer to the
current zone instead of loading it from TLS. Slow ZoneList constructor
was removed and a zone pointer was propagated to its former call sites.

R=vegorov@chromium.org

[Vyacheslav Egorov (Chromium), 2011-07-15 12:20:58]

change LGTM 

but I think we should not commit it unless there is a significant improvement
from it because it pollutes interfaces with unnecessary details of allocation
and introduces tight coupling between objects (like e.g HGraph) and allocation
facilities (Zone).