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`.

Query the object database for multiple objects at a time, given their
object ID (which may be abbreviated) and optional type.

Restricting files to size_t is a silly limitation. The loose backend
writes to a file directly, so there is no issue in using 63 bits for the
size.

We still assume that the header is going to fit in 64 bytes, which does
mean quite a bit smaller files due to the run-length encoding, but it's
still a much larger size than you would want Git to handle.