Make `Lazy.orElseGet(Supplier)` public

[gregturn]

A very useful method would be the orElseGet() method, giving you the ability to provide alternative behavior for a given Lazy but without having to commit to the final object UNLESS invoked.

Optional.orElseGet(Supplier<? extends T> supplier) is quite handy in this respect. Parity in our Lazy type would be quite useful.