The return code description use '-' char as list of return value.
But with the generation of the documentation the char '-' create an
ambiguous return code value who seem an negative value.

Instead of using a signed type (`off_t`) use a new `git_object_size_t`
for the sizes of objects.

Docurium seems to choke on this header because it can't see both
git_indexer_progress & git_indexer_progress_cb, let's add the include.

Update internal usage of `git_transfer_progress` to
`git_indexer_progreses`.

Since we now always build the library with cdecl calling conventions,
our callbacks should be decorated as such so that users will not be able
to provide callbacks defined with other calling conventions.

The `GIT_CALLBACK` macro will inject the `__cdecl` attribute as
appropriate.

git_object_t is the future; update the public API to use it.  This will
also ensure that we can build our tests which make use of the old API
without modification (and without compiler warnings).

The streaming read functionality should provide the length and the type
of the object, like the normal read functionality does.

There are references to odb_backends.h when the file is actually named
odb_backend.h and in the sys folder.

Take (and write to) an array of a struct, `git_odb_expand_id`.