rust: use the bytes-cast crate to parse persistent nodemaps
This crate casts pointers to custom structs, with compile-time safety checks,
for easy and efficient binary data parsing.
See https://crates.io/crates/bytes-cast and
https://docs.rs/bytes-cast/0.1.0/bytes_cast/
Differential Revision: https://phab.mercurial-scm.org/D9788
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