We expose index_length and index_node in the C capsule, so that
the Rust representation of the C index can implement the RevlogIndex
Because our Node is actually a one-field struct, we have
to decorate it for direct FFI exchange with the C char*
It would be a good thing to get a length from the C layer, but doing
so right now would probably interfere with the upcoming changes that
will happen there for the hash length.