chunker: optimized the barrel shift

move the modulo out of the barrel shift and use 32bits variables so
that the compiler recognizes it and uses the "rol*" asm instructions.

Before: 245 MiB/s
After: 338 MiB/s

CPU: Intel(R) Core(TM) i5-3320M CPU @ 2.60GHz

Modification by Jonas: commit message formatting and added
Cyril Roussillon to AUTHORS
crypto: store EVP_CIPHER_CTX on the stack