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/STArray.h

@@ -40,7 +40,7 @@ private:
     list_type v_;
 
 public:
-    // Read-only iteration 
+    // Read-only iteration
     class Items;
 
     static char const* getCountedObjectName () { return "STArray"; }
@@ -135,6 +135,10 @@ public:
     {
         v_.clear ();
     }
+    void reserve (std::size_t n)
+    {
+        v_.reserve (n);
+    }
     void swap (STArray & a) noexcept
     {
         v_.swap (a.v_);