rust-index: add Sync bound to all relevant mmap-derived values All readonly mmaps are Sync as far as Rust is concerned. Integrity of the mmap'ed file is a concern separate to Rust's memory model, since it requires out-of-program handling via locks, etc. This will help when we start sharing the Rust Index with Python.

debugindexstats: handle the lack of Rust support better We don't have any stats in the Rust index. Currently it is not known which stats would be interesting to get, so if they end up being important, we can add them later.