@Nonnull
    static <U> Promise<U> supplyingSync(@Nonnull Supplier<U> supplier) {
        Promise<U> p = empty();
        return p.supplySync(supplier);
    }
Returns a new Promise, and schedules it's population via the given supplier.

@param supplier the value supplier
@param <U> the result type
@return the promise
helper/src/main/java/me/lucko/helper/promise/Promise.java