Changed the generation of type variable names

[rcythr]

Great resource, thanks for putting it out on the web for everyone to learn from.

I thought the type and output of newTyVar was a little ugly, so I wrote up a little patch to change it:

1. It no longer requires an argument, which is quite satisfying looking in the Monad.
2. It produces more friendly TyVar values.

This is mostly an aesthetic thing as I much prefer type variables (a, b, c, d, ..., aa, ab, ...) to (a0, a1, a2, a3, a4, ...). If you don't agree then feel free to reject this PR.

[mgrabmueller]

Thanks for the contribution, I like the change.