Added feedback from Tom on "on nil" semantics.
Merge pull request #973 from vasslitvinov/add-toms-feedback-to--on-nil.future

Added feedback from Tom on "on nil" semantics.

In his email  to me Tom says:

"[...] the discussion focuses on what "on c" means when c is of class type.  As an unsupported opinion, I think that users are likely to find it more useful if "on c" means "go to the locale on which c is allocated".  In the case that c == nil, there is no allocation associated with c, therefore "on c" is undefined.  I would expect a runtime error to result.

I think this is consistent, because instead of using 0 (== rootLocale.id) as the locale_id portion of a default-initialized wide pointer, we could use a distinguished value LOCALE_UNINITIALIZED.  If the address portion of the pointer is zero, no memory has been allocated, so the class object does not exist on any particular locale.  It is, in that sense, absurd to talk about "nil" residing on any particular locale."