Reduce size of encoded array slab and bump version

[fxamacker]

Updates #292

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

⚠️ This change requires data migration and data format version was bumped from 0 to 1.

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• Targeted PR against main branch
• Linked to Github issue with discussion and accepted design OR link to spec that describes this work
• Code follows the standards mentioned here
• Updated relevant documentation
• Re-reviewed Files changed in the Github PR explorer
• Added appropriate labels

[codecov-commenter]

Codecov Report

Merging #330 (1ea371a) into main (24f8172) will increase coverage by 0.21%.
The diff coverage is 69.09%.

@@            Coverage Diff             @@
##             main     #330      +/-   ##
==========================================
+ Coverage   64.87%   65.08%   +0.21%     
==========================================
  Files          14       14              
  Lines        8443     8623     +180     
==========================================
+ Hits         5477     5612     +135     
- Misses       2267     2300      +33     
- Partials      699      711      +12     

Impacted Files	Coverage Δ
array.go	70.54% <68.97%> (+0.14%)	⬆️
array_debug.go	52.28% <100.00%> (ø)

... and 2 files with indirect coverage changes