Differentiate no-std from alloc within tests/no-std

Fixes `no-std` builds for packages which intended to be `no-std` (without
`alloc`).

Updates a variety of MSRVs to 1.73 due to `flexible-transcript` no longer using
`std-shims` to achieve 1.66 (as `std-shims` requires `alloc`). A future
improvement would be for `std-shims` to have an `alloc` feature and only
provide MSRV shims without it.

crypto/dalek-ff-group/src/ciphersuite.rs

@@ -29,7 +29,10 @@ macro_rules! dalek_curve {
       }
 
       fn hash_to_F(dst: &[u8], data: &[u8]) -> Self::F {
-        Scalar::from_hash(Sha512::new_with_prefix(&[dst, data].concat()))
+        let mut digest = Sha512::new();
+        digest.update(dst);
+        digest.update(data);
+        Scalar::from_hash(digest)
       }
     }
   };