rust: add a new binary loose object map format

Our current loose object format has a few problems.  First, it is not
efficient: the list of object IDs is not sorted and even if it were,
there would not be an efficient way to look up objects in both
algorithms.

Second, we need to store mappings for things which are not technically
loose objects but are not packed objects, either, and so cannot be
stored in a pack index.  These kinds of things include shallows, their
parents, and their trees, as well as submodules. Yet we also need to
implement a sensible way to store the kind of object so that we can
prune unneeded entries.  For instance, if the user has updated the
shallows, we can remove the old values.

For these reasons, introduce a new binary loose object map format.  The
careful reader will notice that it resembles very closely the pack index
v3 format.  Add an in-memory loose object map as well, and allow
enabling writing to a batched map, which can then be written later as
one of the binary loose object maps.  Include several tests for round
tripping and data lookup across algorithms.

Note that the use of this code elsewhere in Git will involve some C code
and some C-compatible code in Rust that will be introduced in a future
commit.  Thus, for example, we ignore the fact that if there is no
current batch and the caller asks for data to be written, this code does
nothing, mostly because this code also does not involve itself with
opening or manipulating files.  The C code that we will add later will
implement this functionality at a higher level and take care of this,
since the code which is necessary for writing to the object store is
deeply involved with our C abstractions and it would require extensive
work (which would not be especially valuable at this point) to port
those to Rust.

Signed-off-by: brian m. carlson <sandals@crustytoothpaste.net>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

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