Auto merge of #3491 - Eirik0:3218-sapling-import-export-wallet, r=str4d

Add Sapling support to z_importwallet and z_exportwallet

Includes code adapted from upstream PR https://github.com/bitcoin/bitcoin/pull/8323

Closes #3218.

qa/pull-tester/rpc-tests.sh

@@ -16,6 +16,7 @@ testScripts=(
     'wallet_treestate.py'
     'wallet_anchorfork.py'
     'wallet_changeindicator.py'
+    'wallet_import_export.py'
     'wallet_protectcoinbase.py'
     'wallet_shieldcoinbase.py'
     'wallet_mergetoaddress.py'