Create a preview implementation of the proposed "Stable Values & Collections" API.

# Summary
Introduce a _Stable Values & Collections API_, which provides performant immutable value holders where elements
are initialized _at most once_. Stable Values & Collections offer the performance and safety benefits of
final fields, while offering greater flexibility as to the timing of initialization.

# Goals
 * Provide an easy and intuitive API to describe value holders that can change at most once.
 * Decouple declaration from initialization without significant footprint or performance penalties.
 * Reduce the amount of static initializer and/or field initialization code.
 * Uphold integrity and consistency, even in a multi-threaded environment.

For more details, see the draft JEP: https://openjdk.org/jeps/8312611