Add operator[] field accessors to STObject:

New array index operators allow for concise reading and
writing of fields in the STObject, with associated unit test.

src/ripple/protocol/STVector256.h

@@ -31,6 +31,8 @@ class STVector256
     : public STBase
 {
 public:
+    using value_type = std::vector<uint256> const&;
+
     STVector256 () = default;
 
     explicit STVector256 (SField const& n)
@@ -80,6 +82,20 @@ public:
         return mValue.empty ();
     }
 
+    STVector256&
+    operator= (std::vector<uint256> const& v)
+    {
+        mValue = v;
+        return *this;
+    }
+
+    STVector256&
+    operator= (std::vector<uint256>&& v)
+    {
+        mValue = std::move(v);
+        return *this;
+    }
+
     void
     setValue (const STVector256& v)
     {
@@ -93,15 +109,14 @@ public:
         return mValue;
     }
 
-    // std::vector<uint256> interface:
-    std::vector<uint256>::size_type
+    std::size_t
     size () const
     {
         return mValue.size ();
     }
 
     void
-    resize (std::vector<uint256>::size_type n)
+    resize (std::size_t n)
     {
         return mValue.resize (n);
     }
@@ -124,6 +139,12 @@ public:
         return mValue[n];
     }
 
+    std::vector<uint256> const&
+    value() const
+    {
+        return mValue;
+    }
+
     void
     push_back (uint256 const& v)
     {