Before this change numbits_i64() was incorrect for every value of the
form 0 - 2^x. This only manifested in a visible error if x%7 == 6 (so
for -64, -8192, etc.) at which point `lebsize` would return a value one
too large, causing a panic in commit().
After some discussion with PVH I realise that the repo structure in the
last reorg was very rust-centric. In an attempt to put each language on
a level footing move the rust code and project files into ./rust
