SignerListSet txn and InnerObjectFormats (RIPD-182):

Add support for the SignerListSet transaction as a step toward
multi-sign support.

As part of the SignerListSet implementation, add InnerObjectFormat
templates (similar to TxFormats and LedgerFormats) and enforce them
in STObject, STArray, and STParsedJSON.

src/ripple/protocol/impl/STArray.cpp

@@ -97,6 +97,11 @@ STArray::STArray (SerialIter& sit, SField const& f)
 
         v_.emplace_back(fn);
         v_.back().set (sit, 1);
+        
+        if (v_.back().setTypeFromSField (fn) == STObject::typeSetFail)
+        {
+            throw std::runtime_error ("Malformed object in array");
+        }
     }
 }