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/rpc/impl/TransactionSign.cpp

@@ -195,7 +195,7 @@ bool TxnSignApiFacade::hasAccountRoot () const
     if (!netOPs_) // Unit testing.
         return true;
     return ledger_->exists(
-        getAccountRootIndex(accountID_));
+        keylet::account(accountID_));
 }
 
 error_code_i acctMatchesPubKey (