oidtree: add ability to store data

oidtree: add ability to store data

The oidtree data structure is currently only used to store object IDs,
without any associated data. So consequently, it can only really be used
to track which object IDs exist, and we can use the tree structure to
efficiently operate on OID prefixes.

But there are valid use cases where we want to both:

  - Store object IDs in a sorted order.

  - Associated arbitrary data with them.

Refactor the oidtree interface so that it allows us to store arbitrary
payloads within the respective nodes. This will be used in the next
commit.

Signed-off-by: Patrick Steinhardt <ps@pks.im>
Signed-off-by: Junio C Hamano <gitster@pobox.com>