Making Duration comparable

Solves #754
Which conversion? As a matter of fact just using `Long#compare` is more efficient as it only does 2 comparisons in the worst case than 4 (3 if conditionals and finally `Long#compare`) in the worst case. :wink: 

The method could be reduced to the following implementation and still has the same behaviour:
```java
if (unit == other.unit) {
  return Long.compare(count, other.count);
}

return Long.compare(toNanoseconds(), other.toNanoseconds());
```