Update ChaChaPolyWriteAdapter to bypass crypto when calc'ing len

`ChaChaPolyWriteAdapter` is used to wrap `Writeable` objects in an
AEAD before serializing them. This is great, except that we
sometimes call `<T as ChaChaPolyWriteAdapter>::encode` which first
calls `Writeable::serialized_length` to calculate the length to
pre-allocate in a `Vec` before doing the actual write.

Because `ChaChaPolyWriteAdapter` did not override
`Writeable::serialized_length`, this led to doing the full
cryptographic AEAD encryption and MAC twice, once to calculate the
length and once to actually write the data.

Instead, here, we override
`<ChaChaPolyWriteAdapter as Writeable::serialized_length` to simply
calculate the length we need.

Author: TheBlueMatt

lightning/src/crypto/streams.rs

@@ -51,6 +51,10 @@ impl<'a, T: Writeable> Writeable for ChaChaPolyWriteAdapter<'a, T> {
 
 		Ok(())
 	}
+
+	fn serialized_length(&self) -> usize {
+		self.writeable.serialized_length() + 16
+	}
 }
 
 /// Encrypts the provided plaintext with the given key using ChaCha20Poly1305 in the modified