Add const versions of base_uint.end()/begin(), make size() const.

src/uint256.h

@@ -344,7 +344,17 @@ public:
         return (unsigned char*)&pn[WIDTH];
     }
 
-    unsigned int size()
+    const unsigned char* begin() const
+    {
+        return (unsigned char*)&pn[0];
+    }
+
+    const unsigned char* end() const
+    {
+        return (unsigned char*)&pn[WIDTH];
+    }
+
+    unsigned int size() const
     {
         return sizeof(pn);
     }