docs: add code doctest

Author: gakonst

cast/src/lib.rs

@@ -286,6 +286,21 @@ where
         Ok(self.provider.get_gas_price().await?)
     }
 
+    /// ```no_run
+    /// use cast::Cast;
+    /// use ethers_providers::{Provider, Http};
+    /// use ethers_core::types::Address;
+    /// use std::{str::FromStr, convert::TryFrom};
+    ///
+    /// # async fn foo() -> eyre::Result<()> {
+    /// let provider = Provider::<Http>::try_from("http://localhost:8545")?;
+    /// let cast = Cast::new(provider);
+    /// let addr = Address::from_str("0x00000000219ab540356cbb839cbe05303d7705fa")?;
+    /// let code = cast.code(addr, None).await?;
+    /// println!("{}", code);
+    /// # Ok(())
+    /// # }
+    /// ```
     pub async fn code<T: Into<NameOrAddress> + Send + Sync>(
         &self,
         who: T,