encrypted payment ids are now 64 bit, instead of 256 bit

Pros:
 - smaller on the blockchain
 - shorter integrated addresses

Cons:
 - less sparseness
 - less ability to embed actual information

The boolean argument to encrypt payment ids is now gone from the
RPC calls, since the decision is made based on the length of the
payment id passed.

src/crypto/hash.h

@@ -45,9 +45,13 @@ namespace crypto {
   POD_CLASS hash {
     char data[HASH_SIZE];
   };
+  POD_CLASS hash8 {
+    char data[8];
+  };
 #pragma pack(pop)
 
   static_assert(sizeof(hash) == HASH_SIZE, "Invalid structure size");
+  static_assert(sizeof(hash8) == 8, "Invalid structure size");
 
   /*
     Cryptonight hash functions
@@ -74,3 +78,4 @@ namespace crypto {
 }
 
 CRYPTO_MAKE_HASHABLE(hash)
+CRYPTO_MAKE_COMPARABLE(hash8)