This makes it possible to do zero-copy reading the changelog.i data from
Python to Rust because eventually changelog.i does not have to be fully
accessed and together with mmap, that's at least 100ms perf win.
There is a much more general purposed (support writable, non-contiguous
buffer) PyBuffer implementation in rust-cpython github version .
However, what NodeMap really wants is AsRef<[u8]> and AsRef<[u32]>,
which are not provided by that.
Since the code is short, let's just add our own wrapper. It also allows us
to use the published version of rust-cpython instead of pulling from github.