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/ledger/impl/LedgerMaster.cpp

@@ -661,7 +661,7 @@ public:
     {
         // A new ledger has been accepted as part of the trusted chain
         WriteLog (lsDEBUG, LedgerMaster) << "Ledger " << ledger->getLedgerSeq () << " accepted :" << ledger->getHash ();
-        assert (ledger->peekAccountStateMap ()->getHash ().isNonZero ());
+        assert (ledger->stateMap().getHash ().isNonZero ());
 
         ledger->setValidated();
         ledger->setFull();