Add Tasks.value for making tasks that trivially return a value.

ValueTask appears to be in the test code. I need something like this, and right now I'm having to use Tasks.callable("...", new Callable() { Whatever call() { return value; }); all over the place (actually, a wrapper around that).

I'd like to suggest this change to move it somewhere accessible.  I can provide some examples within LinkedIn source where I think this is a useful thing to do.