rust-index: check that the entry bytes are the same in both indexes
This is a temporary measure to show that both the Rust and C indexes are
kept in sync.
Comes with some related documentation precisions.
For comparison of error cases, see `index_entry_binary()` in `revlog.c`.
edition = "2018"
max_width = 79
wrap_comments = true
error_on_line_overflow = true