public static int compare(int x, int y) {
            return (x < y) ? -1 : ((x == y) ? 0 : 1);
        }
Field mapping implemented on top of hash for field lookup by number.

This is the less efficient than {@code ArrayFieldMap} for almost all cases.
But in case when field numbers are sparse and especially when max field
number is big - this mapping should be used.

@see io.protostuff.runtime.ArrayFieldMap

@author Kostiantyn Shchepanovskyi
protostuff-runtime/src/main/java/io/protostuff/runtime/HashFieldMap.java