minor: comments
Fix a latent problem with onedimensionalize.  It wasn't getting rid of EShapeSize.  It is now.
Partial step.  Properly error out EShape[Size] earlier on. Add a function to compute fold strides, but don't use ityet.
Found and fixed a bugs in SizeAnalysis and ExplicitShapes for segmented ops.  Something else is wrong though, because one of the two new tests in this rev (p20b) seems to be wrong from thevery initial conversion.
The prev bug was actually a SEVERE problem with Acc conversion.  It was not keeping separate environments for scalar and array bindings.