Convert IndexImpl to IdMap<IndexImpl>

[ava57r]

Hello.

Example

let index = FlatIndexImpl::new_l2(4).unwrap();
let id_map = IdMap::new(index).unwrap();
write_index(&id_map, "/tmp/id_map_ex.dat").unwrap();
let new_index = read_index("/tmp/id_map_ex.dat").unwrap();

new_index is IndexImpl

[ava57r]

support in faiss
facebookresearch/faiss#1917

[ava57r]

faiss_IndexIDMap_cast does not work.
IdMap contains pointer for inner index here https://github.com/Enet4/faiss-rs/blob/master/src/index/id_map.rs#L78

Need add getter for Inner index into Faiss C_API.

[ava57r]

add function sub_index facebookresearch/faiss#1982

[ava57r]

a new native faiss release https://github.com/facebookresearch/faiss/releases/tag/v1.7.2