We don't want to provide a generic `map` function, since that would
enable users to arbitrarily alter the value connected to a given cell.
If a new value is being produced, that should either happen outside of
the context of a cell (e.g. intermediate values from witness generation)
or in the context of a newly-assigned cell.
However, in the case of the `Assigned<F>` type, we do need the ability
to evaluate the deferred inversion in some cases (e.g. to then operate
on the bits of the value). So for this `AssignedCell` specialization, we
provide a pass-through `evaluate()` method that otherwise preserves the
cell-value connection.
Jack Grigg