Added some of the suggestions in #10

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
doesn’t
seem there’s any standard operator precedence for these three operators.
Merge pull request #12 from rastus-vernon/master

Added some of the suggestions in #10