Refactor creating new StorableSlab

[fxamacker]

Updates #296 #292 https://github.com/onflow/flow-go/issues/1744

Description

Currently, in order to create StorableSlab, user needs to:

• generate SlabID (used to call StorageID) from storage
• create new StorableSlab
• store created StorableSlab in storage

This commit unexports StorableSlab fields and adds NewStorableSlab().

With this change, user can simply call NewStorableSlab().

---

• [ ] Targeted PR against main branch
• [ ] 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 #324 (37f2201) into fxamacker/unexport-SlabID-fields (33b8013) will decrease coverage by 0.12%. The diff coverage is 38.77%.

@@                         Coverage Diff                          @@
##           fxamacker/unexport-SlabID-fields     #324      +/-   ##
====================================================================
- Coverage                             64.64%   64.53%   -0.12%     
====================================================================
  Files                                    14       14              
  Lines                                  8050     8075      +25     
====================================================================
+ Hits                                   5204     5211       +7     
- Misses                                 2168     2187      +19     
+ Partials                                678      677       -1     

Impacted Files	Coverage Δ
slab.go	100.00% <ø> (ø)
storage.go	74.48% <ø> (+0.31%)	:arrow_up:
storable_slab.go	31.88% <25.00%> (-13.58%)	:arrow_down:
array.go	69.92% <100.00%> (ø)
array_debug.go	52.28% <100.00%> (ø)
encode.go	79.36% <100.00%> (ø)
map.go	66.88% <100.00%> (+0.09%)	:arrow_up:
map_debug.go	51.77% <100.00%> (ø)