PathSet: Add some TYPE_* constants
VariableLength: return buffer directly in toBytes
Vector256: Add removeUnstable method
HalfSha512: Add add(SerializedType st) method
Hash256: Add isZero() and isNonZero() methods
Index: Add ledgerHashes methods
Update ShaMap package
* Add ShaMapDiff class
* Add AccountStateBuilder
* Rename ShaMapInnerNode -> ShaMapInner
* Rename ShaMapLeafNode -> ShaMapLeaf
* Various helper methods
LedgerHeader: Make members public
Update various tests
Add walkPrevious to TransactionMeta