Ok - I learned something new/great. I have no need to use the icedhash library
at all, and can use the "normal" xxhash lib!

I thought that the "wrapper" aspect with nim was wrapping access to
`libxxhash.so` ... but I learned it is really just vendoring in the C code.
This is fine for me, as I really just needed standalone code.

Even the vendored xxhash is ~4x slower (after I compile with extensive
optimzations) than the canonical `xxh64sum` C-based executable... but I'm
thinking that there must be some aggressively shorted start times in `xxh64sum`
that Nim might not do?  I've not unrolled it all to check raw hashing yet.