Some simplifications
UNTESTED: Added support for data flow variables embedded into arrays and
objects. Though I haven't tested this yet, the current test set runs.
MAJOR changes -

1. ":=" now binds lazily. i.e. its RHS won't be forced before binding
the result.

2. Preliminary support for data flow variables embedded into objects
and arrays. This lets you do parallel operations more naturally.
Clarified use of arrays and objects with dfvars