Compute a 2D precedence table from a list of precedences.
PRECS should be a list, sorted by precedence (e.g. "+" will
come before "*"), of elements of the form (left OP ...)
or (right OP ...) or (nonassoc OP ...) or (assoc OP ...). All operators in
one of those elements share the same precedence level and associativity.
(fn PRECS)