* [c_api] leverage index downcast

- make index downcast declaration+definition possible with macros
- use macros on existing index types

* [c_api] Add IndexLSH to C API

* [c_api] Add Index IO interface

- new header index_io_c.h for access to the index_io.h API
- function`faiss_write_index` and overloads for dumping index to a file
- function`faiss_read_index` and overloads for reading index from a file
- function`faiss_clone_index` for cloning an index
- update C example to save index to a file

* [c_api] Add IndexIDMap and IndexShards API