Refactor Ledger and LedgerEntrySet:

Member functions and free functions on Ledger and LedgerEntrySet are
rewritten in terms of new abstract interfaces `BasicView` and `View`,
representing the set of non-decomposable primitives necessary to read
and write state map items in a ledger, and to overlay a discardable
view onto a Ledger that can calculate metadata during transaction
processing. const-correctness is enforced through the parameter and
return types.

The MetaView now supports multi-level stacking: A MetaView can be
stacked on top of either a Ledger or another MetaView, up to any
number of levels.

The getSLEi member function is removed. The CachedView wrapper
replaces it, wrapping a View such that any function called with a
CachedView will go through the SLECache.

* Add BasicView, View, CachedView
* Rename LedgerEntrySet to MetaView
* Factor out free functions
* Consolidate free functions in ViewAPI
* Remove unused class members and free functions

src/ripple/app/tx/impl/SetAccount.cpp

@@ -18,11 +18,12 @@
 //==============================================================================
 
 #include <BeastConfig.h>
-#include <ripple/protocol/Quality.h>
 #include <ripple/app/tx/impl/Transactor.h>
+#include <ripple/ledger/ViewAPI.h>
 #include <ripple/basics/Log.h>
 #include <ripple/core/Config.h>
 #include <ripple/protocol/Indexes.h>
+#include <ripple/protocol/Quality.h>
 #include <ripple/protocol/TxFlags.h>
 
 namespace ripple {
@@ -140,7 +141,8 @@ public:
         //
         if (bSetRequireAuth && !(uFlagsIn & lsfRequireAuth))
         {
-            if (!mEngine->view().dirIsEmpty (getOwnerDirIndex (mTxnAccountID)))
+            if (! dirIsEmpty (mEngine->view(),
+                keylet::ownerDir(mTxnAccountID)))
             {
                 m_journal.trace << "Retry: Owner directory not empty.";
                 return (mParams & tapRETRY) ? terOWNERS : tecOWNERS;