The experimental function signature is only available when
`GIT_EXPERIMENTAL_SHA256` is enabled.

Teach the loose object database how to cope with SHA256 objects.

Move the arguments to `git_odb_loose` into an options structure.

Allow the object database to take an oid type that it supports.  This
oid type will be used to validate the objects that the backends provide.

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.

We've had an fsync option for a long time, but it was "ignored".
Stop ignoring it.