Make Lazy.filter lazy

[danieldietrich]

See also #1861 and this discussion.

- public Option<T> filter(Predicate<? super T> predicate) {
-     final T v = get();
-     return predicate.test(v) ? Option.some(v) : Option.none();
+ public Lazy<Option<T>> filter(Predicate<? super T> predicate) {
+    Objects.requireNonNull(predicate, "predicate is null");
+    return map(t -> Option.some(t).filter(predicate));
}

However, there is one thing that currently hinders us from doing so: The return type is not compatible with Value.filter anymore.

Our options are:

• leave Lazy.filter as-is (i.e. not performing this change)
• change Value.filter (or remove it, like flatMap)

Update: No problem! Value does not have filter anymore :)