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/streams.h

@@ -308,13 +308,6 @@ public:
             s.write((char*)&vch[0], vch.size() * sizeof(vch[0]));
     }
 
-    template<typename T>
-    unsigned int GetSerializeSize(const T& obj)
-    {
-        // Tells the size of the object if serialized to this stream
-        return ::GetSerializeSize(obj, nType, nVersion);
-    }
-
     template<typename T>
     CDataStream& operator<<(const T& obj)
     {
@@ -457,13 +450,6 @@ public:
             throw std::ios_base::failure("CAutoFile::write: write failed");
     }
 
-    template<typename T>
-    unsigned int GetSerializeSize(const T& obj)
-    {
-        // Tells the size of the object if serialized to this stream
-        return ::GetSerializeSize(obj, nType, nVersion);
-    }
-
     template<typename T>
     CAutoFile& operator<<(const T& obj)
     {