stream-cipher: remove NewBlockCipher bound on FromBlockCipher

Note: breaking change.

As far as I can tell this was an oversight: `FromBlockCipher` never
needs to instantiate a new block cipher (isn't that the point?),
making this bound needlessly limiting.

The rationale for it in the first place appears to be for the blanket
impl of `NewStreamCipher` for `C: FromBlockCipher` which needs it, but
we can add the bound to the blanket impl, rather than (needlessly)
sticking it on the trait.

stream-cipher/src/lib.rs

@@ -173,7 +173,7 @@ impl<C: SyncStreamCipher> SyncStreamCipher for &mut C {
 #[cfg_attr(docsrs, doc(cfg(feature = "block-cipher")))]
 pub trait FromBlockCipher {
     /// Block cipher
-    type BlockCipher: BlockCipher + NewBlockCipher;
+    type BlockCipher: BlockCipher;
     /// Nonce size in bytes
     type NonceSize: ArrayLength<u8>;
 
@@ -188,6 +188,7 @@ pub trait FromBlockCipher {
 impl<C> NewStreamCipher for C
 where
     C: FromBlockCipher,
+    C::BlockCipher: NewBlockCipher,
 {
     type KeySize = <<Self as FromBlockCipher>::BlockCipher as NewBlockCipher>::KeySize;
     type NonceSize = <Self as FromBlockCipher>::NonceSize;