Document the Alternative benchmark
Fix a huge performance regression under GHC 7.8.1

This improves runtime of benchmarks/Alternative.hs from 12 seconds
to 0.04 (the same as with GHC 7.6), and thus fixes gh-56.

--HG--
extra : amend_source : c9e91a509a0f611019bbbc60ae5ea25430522e97
Bump version to 0.11.2.1, and update included files
Added tag 0.11.2.1 for changeset 48809d771646