Name of binary encoding scheme for integer numbers
The technique idea is perfectly described in Yuval Filmus answer. Even if slightly different, it is called Truncated binary encoding in Wikipedia. I couldn't find an original source for that, apart from a mention in a patent, in this book, or in this Google API
Another mention can be found in the ACM ICPC 2011–2012, Northeastern European Regional Contest, November 27, 2011.