implement mapToEntryPartial

[rhuffy]

Follows #183 and 78c0490, which introduced

StreamEx.mapPartial, EntryStream.mapToKeyPartial/mapToValuePartial/mapKeyValuePartial

This PR implements EntryStream.mapToEntryPartial, which takes a partial value mapper Function<K, Optional<V>> and returns an EntryStream<K, V> with the empty optionals removed.

The intention is to replace

StreamEx.of(x).mapToEntry(partialFunction).flatMapValues(Optional::stream);

with

StreamEx.of(x).mapToEntryPartial(partialFunction);