Added some of the suggestions in #10

I have not added the silly suggestions in #10, and have not added the square root symbol because it has the same problem as the negation symbol: you’d like to use it as `√(9 +√49)` but you must write `(√)(9 + ((√) 49))`. This does not mean they should not be added, but I have omitted them since they are questionable.

DIVIDES and DOES NOT DIVIDE: infix 4, because they’re non-associative
and all the non-associative operators are infix 4.

XOR, NAND and NOR: infixr 3 because that’s the fixity of && and it doesn’t
seem there’s any standard operator precedence for these three operators.

I have not tested these additions, so it is possible there’s a mistake somewhere. However, GHC was able to compile it (according to Travis CI).