Implement basic Sapling v4 transaction parser

Details of Sapling datatypes will be filled in later; for now, they are treated
as binary blobs.

Includes code cherry-picked from upstream commit:
    7030d9eb47254499bba14f1c00abc6bf493efd91
    BIP144: Serialization, hashes, relay (sender side)

src/streams.h

@@ -22,6 +22,53 @@
 #include <utility>
 #include <vector>
 
+template<typename Stream>
+class OverrideStream
+{
+    Stream* stream;
+
+    const int nType;
+    const int nVersion;
+
+public:
+    OverrideStream(Stream* stream_, int nType_, int nVersion_) : stream(stream_), nType(nType_), nVersion(nVersion_) {}
+
+    template<typename T>
+    OverrideStream<Stream>& operator<<(const T& obj)
+    {
+        // Serialize to this stream
+        ::Serialize(*this, obj);
+        return (*this);
+    }
+
+    template<typename T>
+    OverrideStream<Stream>& operator>>(T&& obj)
+    {
+        // Unserialize from this stream
+        ::Unserialize(*this, obj);
+        return (*this);
+    }
+
+    void write(const char* pch, size_t nSize)
+    {
+        stream->write(pch, nSize);
+    }
+
+    void read(char* pch, size_t nSize)
+    {
+        stream->read(pch, nSize);
+    }
+
+    int GetVersion() const { return nVersion; }
+    int GetType() const { return nType; }
+};
+
+template<typename S>
+OverrideStream<S> WithVersion(S* s, int nVersion)
+{
+    return OverrideStream<S>(s, s->GetType(), nVersion);
+}
+
 /** Double ended buffer combining vector and stream-like interfaces.
  *
  * >> and << read and write unformatted data using the above serialization templates.