* Have add_gate take a generic type instead of GateRef
There are a couple advantages
- Users writing their own gates won't need to know about the `GateRef` wrapper; it's more of an internal thing now.
- Easier access to gate methods requiring `self` -- for example, `split_le_base` can just call `gate_type.limbs()` now.
* Update comment
* Always insert
With this approach, we don't need `Target::PublicInput`; any routable `Target` can be marked as a public input via `register_public_input`. The circuit itself hashes these targets, and routes the hash output to the first four wires of a `PublicInputGate`, which is placed at an arbitrary location in the circuit.
All gates have direct access to the purported hash of public inputs. We could think of them as accessing `PI_hash_i(x)` (as in Plonk), but these are now (four) constant functions, so they effectively have direct access to the hash itself.
`PublicInputGate` checks that its first four wires match this purported public input hash. The other gates ignore the hash.
Resolves#64.
