Fix slab size when resetting mutable storable in OrderedMap

[fxamacker]

Updates #292 #335

Changes

Recompute slab size by adding all element sizes instead of using the size diff of old and new element because oldElem can be the same storable when the same value is reset and oldElem.ByteSize() can equal storable.ByteSize().

Given this, size diff of the old and new element can be 0 even when its actual size changed.

Also extracted map data slab prefix computation to a new function.

This is prep work for atree inlining to prevent a bug from surfacing.

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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

Patch coverage: 100.00% and project coverage change: +0.04% 🎉

Comparison is base (dda0495) 65.32% compared to head (cc19ec9) 65.36%.

Additional details and impacted files

@@                                        Coverage Diff                                        @@
##           fxamacker/fix-slab-size-when-resetting-same-storable-in-array     #337      +/-   ##
=================================================================================================
+ Coverage                                                          65.32%   65.36%   +0.04%     
=================================================================================================
  Files                                                                 14       14              
  Lines                                                               8773     8784      +11     
=================================================================================================
+ Hits                                                                5731     5742      +11     
  Misses                                                              2325     2325              
  Partials                                                             717      717              

Files Changed	Coverage Δ
map.go	67.54% <100.00%> (+0.09%)	⬆️

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