Review file hashing strategy

We mmap and hash. It doesn't seem to be particularly fast for large files (at leasts timings on macos between `b0-hash` and [this test](https://github.com/dbuenzli/bytesrw/blob/main/test/xxh3tap.ml) on a 53GB file are respectively 1:46.41 and 11.8s and obviously with quite a different memory profile).