multiple return values - TensorList + PrimitiveRWList version

[ ] modify multi-tensor eval so that inference works at usage site
[ ] reduce usage of list overloading to improve compilation times and error messages
[ ] consider using built-in types over custom versions e.g. Pair over custom vect-like types, and a built-in right- (or left-)erased type if possible
[ ] choose location of custom vect-like types if using them
[ ] is ShapeIndex (the Idris version) constructed in the right place? Would passing a List Nat to the Idris XLA API work better?

I may want to change more than this

joelberkeley / spidr