Remove some Haskell operators not present in Idris

Also removed the logically questionable mapping of () (Unit) to ∅.
≡ for == might be questionable, too.  A lot of this, like `>-`, aren't present in the Idris REPL and I don't even know what they are.  Things like `sqrt`, `pi`, and `notElem`, aren't (currently) present either, but might be at some point
Merge pull request #1 from LeifW/patch-1

Remove some Haskell operators not present in Idris