Lift the Default bound for the Circuit trait

The `Compiler::compile` function keeps this bound since it actually
requires the circuit to have a `Default` implementation, but all other
functionality now works without said bound.

Resolves #715

src/composer/circuit.rs

@@ -11,7 +11,7 @@ use super::Composer;
 /// Circuit implementation that can be proved by a Composer
 ///
 /// The default implementation will be used to generate the proving arguments.
-pub trait Circuit: Default {
+pub trait Circuit {
     /// Circuit definition
     fn circuit<C>(&self, composer: &mut C) -> Result<(), Error>
     where

src/composer/compiler.rs

@@ -26,7 +26,7 @@ impl Compiler {
         label: &[u8],
     ) -> Result<(Prover<C>, Verifier<C>), Error>
     where
-        C: Circuit,
+        C: Default + Circuit,
     {
         Self::compile_with_circuit(pp, label, &Default::default())
     }