simplify precedence/associativity tables:
 - instead of having the values just be precedences, and using
   a separate set to record right-associativity, have the values
   include associativity as well (plus the second operator part
   of mixfix operators)

advantages include:
 - less confusing and easier to find the information
 - more clear exactly which operator the associativity refers to
 - allows easy extension to add in non-associative operators
tests to confirm that associativity is irrelevant if precedence is different