Candidate refactor for Unlit.hs

My crack at https://github.com/idris-lang/Idris-dev/issues/1593.

It's pretty much just refactoring, since that's what was requested, however there has been one functional change: It no longer treats ">" and "> " prefixes as identical, instead maintaining their indentation difference.

This is in keeping with the literate haskell translation style. Code is extracted from ">" prefixed lines by simply converting the ">" into whitespace.

Reference: https://www.haskell.org/onlinereport/syntax-iso.html