For almost all the sponge constructions defined in the Poseidon paper,
the domain can be defined completely statically. Variable-length hashing
requires knowledge of the message length, but that can be provided to
the fixed padding function in a subsequent commit, and in any case we
can't use variable-length inputs in a circuit.
As the underlying `Region` methods now return `AssignedCell` instead of
`Cell`, we can simplify all the places where we then constructed a
`CellValue` struct.
We change `CellValue` into a typedef of `AssignedCell` to simplify the
migration in this commit.
The migration from `CellValue` to `AssignedCell` requires several other
changes:
- `<CellValue as Var>::value()` returned `Option<F>`, whereas
`AssignedCell::<F, F>::value()` returns `Option<&F>`. This means we
need to dereference, use `Option::cloned`, or alter functions to take
`&F` arguments.
- `StateWord` in the Poseidon chip has been changed to a newtype around
`AssignedCell` (the chip was written before `CellValue` existed).
Jack Grigg
ying tong