Reduce encoded array slab size and bump version

Changes:
- bump Atree format version to 1 (was 0)
- remove redundant version and flag data from root slabs
  (2 bytes per root slab)
- remove redundant slab address from children headers in metadata slab
  since all child slabs have the same address
  (8 bytes per child header per metadata slab)
- reduce encoded child slab size in children headers in metadata slab
  from 4 bytes to 2 bytes
  (2 bytes per child header per metadata slab)
- decode version 0 and 1
- encode only version 1
- add tests to decode array slabs data in version 0