Move git_config_backend to include/git2/sys

Moving backend implementor objects into include/git2/sys so the
APIs can be isolated from the ones that normal libgit2 users
would be likely to use.
Move odb_backend implementors stuff into git2/sys

This moves some of the odb_backend stuff that is related to the
internals of an odb_backend implementation into include/git2/sys.

Some of the stuff related to streaming I left in include/git2
because it seemed like it would be reasonably needed by a normal
user who wanted to stream objects into and out of the ODB.

Also, I added APIs for traversing the list of backends so that
some of the tests would not need to access ODB internals.
Added git_repository_new function
Move some low-level repo fns to include/git2/sys
Added git_commit_create_oid
Move git_commit_create_from_oids into sys/commit.h

Actually this renames git_commit_create_oid to
git_commit_create_from_oids and moves the API declaration to
include/git2/sys/commit.h since it is a dangerous API for general
use (because it doesn't check that the OID list items actually
refer to real objects).