First attempt to generalize ParVec to a family of splittable types.
This is running into trouble right now because the WithSession approach
relies on a type function which isn't known to be injective, so casting
becomes a problem.
WIP: Playing around with the opposite approach from the gparsplit1 branch.
That is, exposing the 's' param rather than recovering it through a type function.
New version of ParST with some lame wrappers for flipping order of type args.
Simple rename + minor
Add a specialized version of ParST for one boxed vector
A variation of ST/Vec forthe two-vector case
Merge branch 'gparsplit2'