Release 5.9

• toMap(mapSupplier) BiCollector with null support (works around https://bugs.openjdk.java.net/browse/JDK-8148463).
• Substring splitKeyValuesAround():

      first(',')
          .repeatedly()
          .splitKeyValuesAround(first('='), "k1=v1,k2=v2")

• asMap(Struct): protobuf Struct to POJO.
• fromValue(Value): protobuf Value to POJO.