* Redesign Sapling data model for V5 shared anchor and spends
The shared anchor is only present if there are any spends.
As part of this change, delete the manual PartialEq impl and its tests,
because we can derive PartialEq now.
* Stop creating a temporary Vec for the spend and output iterators
* Rename TransferData variants
Interactive rename using the following commands:
```sh
fastmod Spends SpendsAndMaybeOutputs
fastmod NoSpends JustOutputs
```
* Refactor out common sprout nullifier code
* Implement the AtLeastOne constrained vector type
This vector wrapper ensures that it always contains at least one element.
* Simplify Sapling TransferData using AtLeastOne
Also update the RFC to use AtLeastOne for Orchard.