"add new 64-bit btree types (patch from Jim), merged from trunk"

Even though, as noted in the history file, there is no official release
of standalone ZODB 3.7, it corresponds with the release of Zope 3.3 and
2.10 in 2006, and should be considered feature-frozen.  Jim has
requested that these changes be reserved for the trunk (to be ZODB 3.8).

A subsequent checkin will clarify the status of 3.7 in HISTORY.txt.