Add mapKeysPartial and mapValuesPartial to EntryStream

[roblburris]

When operating over an EntryStream, a common pattern is to call a mapping function on the keys or values that returns an Optional<K> or Optional<V>. As is, one can remove empty Optionals by using flatMap(Keys|Values)(Optional::stream) or mapTo(Key|Value)Partial(partialFunction):

EntryStream.of(x).mapKeys(partialFunction).flatMapKeys(Optional::stream);
// OR
EntryStream.of(x).mapToKeyPartial((key, _value) -> partialFunction(key));

The former introduces an extra allocation for every entry, while the latter isn't the most ergonomic solution.

As a result, this PR implements EntryStream.mapKeysPartial and EntryStream.mapValuesPartial, which takes a partial key/value mapper Function<K, Optional<KK>> and returns an EntryStream<KK, V> with the empty optionals removed. This makes it possible to now do the following:

EntryStream.of(x).mapKeysPartial(partialFunction);

[amaembo]

Thanks for your contribution