Log Message:
afl: Upgrade to afl-2.56b

Switch to GitHub as new upstream.

Upstream changelog
==================
Version 2.56b (2019-09-26)

    Fixed the mismatch between the released version and the version in the code.

Version 2.55b (2019-09-19)

    Exporting more metrics (peak_rss_mb, slowest_exec_ms).

    Improved tests.

    Fixes for bugs and changes in upstream llvm/clang.

Version 2.53b (2019-07-25)

    No functional changes. Updated some comments and license headers to comply \ 
with the open sourcing guidelines and publish the source code on GitHub.