wallet: Switch from SaplingSpendingKey to SaplingExtendedSpendingKey

The wallet now only stores Sapling extended spending keys, and thus can
only be used with keys generated from an HDSeed via ZIP 32.

Note that not all Sapling keys in the wallet will correspond to the
wallet's HDSeed, as a standalone Sapling xsk can be imported via
z_importkey. However, it must have been generated from a seed itself,
and thus is more likely to be backed up elsewhere.

src/wallet/rpcwallet.cpp

@@ -21,6 +21,7 @@
 #include "primitives/transaction.h"
 #include "zcbenchmarks.h"
 #include "script/interpreter.h"
+#include "zcash/zip32.h"
 
 #include "utiltime.h"
 #include "asyncrpcoperation.h"