Improvements to STParsedJSON:

* Cleanups and reduction of copying
* Add STArray::back, operator[], push_back(&&)
* Add make_stvar
* Rework STParsedJSON
* Fix code and unit tests that use STParsedJSON
* STTx move constructor

src/ripple/protocol/STArray.h

@@ -71,11 +71,37 @@ public:
         return emplace(n, buf, std::move(*this));
     }
 
-    void push_back (const STObject & object)
+    STObject& operator[] (std::size_t j)
+    {
+        return v_[j];
+    }
+
+    STObject const& operator[] (std::size_t j) const
+    {
+        return v_[j];
+    }
+
+    STObject& back() { return v_.back(); }
+
+    STObject const& back() const { return v_.back(); }
+
+    template <class... Args>
+    void
+    emplace_back(Args&&... args)
+    {
+        v_.emplace_back(std::forward<Args>(args)...);
+    }
+
+    void push_back (STObject const& object)
     {
         v_.push_back(object);
     }
 
+    void push_back (STObject&& object)
+    {
+        v_.push_back(std::move(object));
+    }
+
     iterator begin ()
     {
         return v_.begin ();
@@ -101,11 +127,6 @@ public:
         return v_.size ();
     }
 
-    STObject& back ()
-    {
-        return v_.back ();
-    }
-
     bool empty () const
     {
         return v_.empty ();