new functions in struct git_config_backend:
* iterator_new(...)
* iterator_free(...)
* next(...)
The old callback based foreach style can still be used with `git_config_backend_foreach_match`
The GIT_CONFIG_LEVEL constants actually work well as an enum
because they are mutually exclusive, so this adds a typedef to
the enum and uses that everywhere that one of these constants are
expected, instead of the old code that typically used an unsigned
int.
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.
