@Override
    public List<Field<T>> getFields()
    {
        return fields;
    }
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