STVar: optimized storage for STObject (RIPD-825):

This introduces the STVar container, capable of holding any STBase-derived
class and implementing a "small string" optimization. STObject is changed
to store std::vector<STVar> instead of boost::ptr_vector<STBase>. This
eliminates a significant number of needless dynamic memory allocations and
deallocations during transaction processing when ledger entries are
deserialized. It comes at the expense of larger overall storage requirements
for STObject.

src/ripple/protocol/STVector256.h

@@ -27,7 +27,7 @@
 
 namespace ripple {
 
-class STVector256 final
+class STVector256
     : public STBase
 {
 public:
@@ -41,6 +41,20 @@ public:
         : mValue (vector)
     { }
 
+    STVector256 (SerialIter& sit, SField::ref name);
+
+    STBase*
+    copy (std::size_t n, void* buf) const override
+    {
+        return emplace(n, buf, *this);
+    }
+
+    STBase*
+    move (std::size_t n, void* buf) override
+    {
+        return emplace(n, buf, std::move(*this));
+    }
+
     SerializedTypeID
     getSType () const override
     {
@@ -50,10 +64,6 @@ public:
     void
     add (Serializer& s) const override;
 
-    static
-    std::unique_ptr<STBase>
-    deserialize (SerialIter& sit, SField::ref name);
-
     Json::Value
     getJson (int) const override;
 
@@ -72,12 +82,6 @@ public:
         mValue = v.mValue;
     }
 
-    std::unique_ptr<STBase>
-    duplicate () const override
-    {
-        return std::make_unique<STVector256>(*this);
-    }
-
     /** Retrieve a copy of the vector we contain */
     explicit
     operator std::vector<uint256> () const