Implicit fmap

I suspect that uniformly applying an expression to each line is going to
be a very common use case, so I made it easy. Just write your function,
we'll use its type to figure out whether we want to apply it to each
line or to the full input!
Clarification: HSL is not my project

Many people seem to think that I am the author of HSL. This is not true.
In this README update, I set things straight by linking to the original
HSL repo, and by explaining how my experimental branch differs from that
original (and from other similar tools).