Make GetSerializeSize a wrapper on top of CSizeComputer

Given that in default GetSerializeSize implementations created by
ADD_SERIALIZE_METHODS we're already using CSizeComputer(), get rid
of the specialized GetSerializeSize methods everywhere, and just use
CSizeComputer. This removes a lot of code which isn't actually used
anywhere.

For CCompactSize and CVarInt this actually removes a more efficient
size computing algorithm, which is brought back in a later commit.

src/uint256.h

@@ -78,11 +78,6 @@ public:
         return sizeof(data);
     }
 
-    unsigned int GetSerializeSize(int nType, int nVersion) const
-    {
-        return sizeof(data);
-    }
-
     uint64_t GetUint64(int pos) const
     {
         const uint8_t* ptr = data + pos * 8;