hg-core: make `Index` owner of its bytes (D8958#inline-14994 followup 1/2) Prevent building `Index` every time it is needed. It was a bad idea anyway. When `Index::new` will return `Result` it will avoid things like `Revlog::len` returning `Result<usize>` instead of `usize`. [X] make `Index` owner of its bytes [ ] make `Index::new` return an error if `offset != bytes.len()` Differential Revision: https://phab.mercurial-scm.org/D9106