Lepton image compression: saving 22% losslessly from images at 15MB/s

Introduced Lepton, a new open-source streaming image compression format. Lepton achieves 22% lossless savings on JPEGs by predicting JPEG block coefficients and feeding these predictions into an arithmetic coder. It offers compression at 5MB/s and decompression at 15MB/s with low memory usage. Lepton has saved Dropbox multiple petabytes of space. Security is enhanced through seccomp, and determinism is rigorously tested. The format encodes AC coefficients using unary, sign, and standard binary, and optimizes DC coefficient compression by predicting them based on neighboring blocks and gradients.