UDT* value subclasses for Flyweight object pools

[willhains]

Currently, the UDT* value classes are subclassed by Pure* and Recyclable* classes.

The Recyclable* classes reuse existing instances, resetting their raw value on every "allocation".

Use-Case

There are use-cases where the same, small number of values are repeated many times.
In these cases, it might be better to keep a cache of previously created instances, similar to String.intern(). (And the UDTString subclass could actually intern the raw value as well.)

Proposed API

public final @Value class CityName extends FlyweightString<CityName> {...}