VectTerm.toStream
A multiplex operation to present changes across a VectTerm as a stream of (K,V) changes.
effecively the opposite of the Stream.by function

EnvTermBuilder.lazyVect
utility to build a vector of 'lazy' streams, that are inflated when retrieved. Useful for binding existing factories, oreasy integration with existing Mekon code libraries
Now trying to evaporate UncollapsedGroup
This will allow me to treat different aggregation containers with typeclasses and implicits, rather than inheritance.
This should allow structurally compatible aggregation classes to be used with no 'wrapping'.
e.g. direct aggregation into a Set[X] because it has an add(X) method.
this should remove some mystery from the aggregation functions, and clean up dead code