Add a dedicated crate for testing ff/group implementors

Provides extensive testing for dalek-ff-group and ed448.

Also includes a fix for an observed bug in ed448.

crypto/ff-group-tests/src/lib.rs

@@ -0,0 +1,9 @@
+#![cfg_attr(docsrs, feature(doc_auto_cfg))]
+
+/// Tests for the Field trait.
+pub mod field;
+/// Tests for the PrimeField and PrimeFieldBits traits.
+pub mod prime_field;
+
+/// Tests for the Group and GroupEncoding traits.
+pub mod group;