In contradiction with Jean-Paul's guidelines on not using Rust due
to lack of knowledge about it inside Nexedi, I am using it here
because it is the fastest way for me to get a working standalone
static binary, I know that language best. Considering we must be
getting results ASAP, this is the best strategy for me. We may
later rewrite it in another language if necessary.

A shell script is included to build the static binary, you need
to install rustup to get rust for musl, an alternative libc that
allows to create real static binaries that embed libc itself too.

Rustup can be found at: https://rustup.rs/

You can get a musl toolchain with:
  $ rustup target add x86_64-unknown-linux-musl

The acl library is being downloaded and built as a static library
by the script, and the rust build system will also build a vendored
copy of openssl as a static library.

Parallel hashing is done a bit differently in that Rust version,
only files contained in the currently processed directories will be
hashed in parallel. If there is a single big file in a directory
hashing will be stuck on that file until it's done and it goes onto
the next directory. To clarify, each file is only hashed on a
single thread, the Python version also does this, it just keeps the
number of files being hashed in parallel to a constant number as
long as there is more files to process, this version will only hash
with one thread per file in the currently processed directory. It
was done that way for sake of simplicity but we can implement an
offload threadpool to mimick what was done in Python later on.