Padmé: Efficiently hiding file sizes

Padmé is a padding algorithm. It hides efficiently the true size of objects (files or network messages). Padmé is designed to work even when objects have very different sizes, and notably outperforms trivial padding approaches. Its overhead is at most +12%, and this number decreases with increasing object size: at most +6% for a 1Mb object, at most +3% for a 1Gb object.