some changes to get unit tests to pass

src/gates/add_many_u32.rs

@@ -92,7 +92,7 @@ impl<F: RichField + Extendable<D>, const D: usize> Gate<F, D> for U32AddManyGate
     }
 
     fn serialize(&self, dst: &mut Vec<u8>) -> IoResult<()> {
-        dst.write_usize(self.num_addends);
+        dst.write_usize(self.num_addends)?;
         dst.write_usize(self.num_ops)
     }
 

src/serialization.rs

@@ -14,7 +14,6 @@ impl WriteU32 for Vec<u8> {
     }
 }
 
-#[cfg(feature = "std")]
 pub trait ReadU32 {
     fn read_target_u32(&mut self) -> IoResult<U32Target>;
 }