[crypto] Split crypto_algorithm into {digest,cipher,pubkey}_algorithm

The various types of cryptographic algorithm are fundamentally
different, and it was probably a mistake to try to handle them via a
single common type.

pubkey_algorithm is a placeholder type for now.

src/crypto/chap.c

@@ -42,7 +42,7 @@
  * eventually be freed by a call to chap_finish().
  */
 int chap_init ( struct chap_response *chap,
-		struct crypto_algorithm *digest ) {
+		struct digest_algorithm *digest ) {
 	size_t state_len;
 	void *state;