add IsString instance for Atom
add IsString instance for Atom

Ignore-this: 8870a7c5146affb1d9851433267d87d5

darcs-hash:20120126123532-1a7c6-4a08d78f42d2a343131a9afadedd8d9c96ae640f

kirbyfan64/jhc,kirbyfan64/jhc,kirbyfan64/jhc,kirbyfan64/jhc