First iteration of circuits naming convention

In this commit I remove all `*Gadget` types and instead implement the naming convention defined here https://github.com/0xPARC/pod2/issues/181#issuecomment-3051954321

The biggest changes can be summarized by:
- a) Removal of `*Gadget` types and their `eval_*` methods in favour of `verb_object_circuit` functions.
- b) The above functions don't create targets that need to be witness-assigned later.  Instead they receive those as arguments.  This clearly shows what's the circuit input and output.

I'm specially happy about the changes from b), I think they make the flow of data in the circuit more clear.

Missing things that I did not address in this PR
- The RecursiveCircuit still uses some old naming conventions like `build`.
- We have some `*Target` types that have methods that define constraints.  I think we can keep those as they are convenient and I don't see them as strongly breaking the new convention: I see them as the object-oriented way to apply the convention.  In those cases the `object` can be omitted from the method when it's implied by the type name, and the `_circuit` suffix doesn't appear because it's implied by the fact that the type is a `*Target`.  Examples are: `SignatureTarget::verify -> BoolTarget`, `StatementTarget::has_native_type -> BoolTarget` or `OperationTypeTarget::as_custom -> (BoolTarget, HashOutTarget, Target)`.

src/backends/plonky2/emptypod.rs

@@ -20,7 +20,7 @@ use crate::{
         basetypes::{Proof, C, D},
         circuits::{
             common::{Flattenable, StatementTarget},
-            mainpod::{CalculateIdGadget, PI_OFFSET_ID},
+            mainpod::{calculate_id_circuit, PI_OFFSET_ID},
         },
         deserialize_proof,
         error::{Error, Result},
@@ -52,10 +52,7 @@ impl EmptyPodVerifyCircuit {
             &self.params,
             &builder.constants(&type_statement().to_fields(&self.params)),
         );
-        let id = CalculateIdGadget {
-            params: self.params.clone(),
-        }
-        .eval(builder, &[type_statement]);
+        let id = calculate_id_circuit(&self.params, builder, &[type_statement]);
         let vds_root = builder.add_virtual_hash();
         builder.register_public_inputs(&id.elements);
         builder.register_public_inputs(&vds_root.elements);