Add PersistentSlabStorage.Deltas()

[fxamacker]

Description

Add PersistentSlabStorage.Deltas() to return number of uncommitted slabs. It can be used by Cadence for memory metering.

Closes #263

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

[codecov-commenter]

Codecov Report

Merging #264 (4fcf12a) into main (2f2648f) will increase coverage by 0.00%. The diff coverage is 76.92%.

@@           Coverage Diff           @@
##             main     #264   +/-   ##
=======================================
  Coverage   69.05%   69.06%           
=======================================
  Files          14       14           
  Lines        7245     7257   +12     
=======================================
+ Hits         5003     5012    +9     
- Misses       1610     1612    +2     
- Partials      632      633    +1     

Impacted Files	Coverage Δ
storage.go	79.54% <76.92%> (-0.10%)	:arrow_down:

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