Nulls

[Brian Goetz]

> I like these rules.  The default-bearing version(s) can be emulated with
> the Option version with an additional Option2Default transform stage, so
> I would like to avoid the default-bearing version(s) if we can get away
> with it.

Right.  We can factor out the default-injection by having an ifEmpty op:

   list.filter(...)
       .ifEmpty(defaultValue)
       .optionBearingOp();

Not a big deal currently as we have relatively few Option-bearing ops, 
but probably better to factor these out rather than ad-hoc fusing. 
Similarly, we can have a filterNulls() op if users want the first 
non-null value:

   list.blah(...)
       .filterNulls()
       .findFirst();

(For stream sources where we *know* there are no nulls, such as 
TreeSet.keys(), the cost of the filterNulls() is O(1) since it can be 
optimized out of existence when we build the chain.)