@Override
    public Field<T> getFieldByName(String fieldName)
    {
        return fieldsByName.get(fieldName);
    }
Field mapping implemented on top of java array for lookup by number.

This is the most efficient implementation for almost all cases. But
it should not be used when field numbers are sparse and especially
when max field number is big - as this mapping internally uses array
of integers with size equal to max field number. In latter case
{@code HashFieldMap} should be used.

@see io.protostuff.runtime.HashFieldMap

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