Use slices to references not slices of concrete objects in pub API

Use slices to references not slices of concrete objects in pub API

Because the C bindings maps objects into new structs which contain
only a pointer to the underlying (immovable) Rust type, it cannot
create a list of Rust types which are contiguous in memory. Thus,
in order to allow C clients to call certain Rust functions, we have
to use &[&Type] not &[Type]. This commit fixes this issue for the
get_route function.