Add a callback to the Encoder

[SupunS]

Work towards https://github.com/dapperlabs/cadence-private-issues/issues/64

Description

It can be useful to have a callback function that gets called during the encoding (e.g: https://github.com/dapperlabs/cadence-private-issues/issues/64), to keep track of the operation.

This PR adds a generic Callback field to the Encoder, which wraps different callback functions. Users of the encoder can register callbacks when they create the encoder. For now, the callback only has one function BeforeEncode which is/should be invoked at the start of each value encoding.

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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
• [ ] Updated relevant documentation
• [x] Re-reviewed Files changed in the Github PR explorer
• [x] Added appropriate labels

[codecov-commenter]

Codecov Report

Merging #287 (7ad3854) into main (d5384d9) will increase coverage by 0.02%. The diff coverage is 100.00%.

@@            Coverage Diff             @@
##             main     #287      +/-   ##
==========================================
+ Coverage   69.08%   69.10%   +0.02%     
==========================================
  Files          14       14              
  Lines        7306     7311       +5     
==========================================
+ Hits         5047     5052       +5     
- Misses       1624     1627       +3     
+ Partials      635      632       -3     

Impacted Files	Coverage Δ
encode.go	80.64% <ø> (ø)
array_debug.go	54.16% <100.00%> (ø)
basicarray.go	53.55% <100.00%> (ø)
map_debug.go	55.50% <100.00%> (ø)
storable.go	68.18% <100.00%> (+2.60%)	:arrow_up:
storage.go	79.30% <100.00%> (+0.52%)	:arrow_up:
array.go	76.15% <0.00%> (-0.34%)	:arrow_down:
map.go	71.78% <0.00%> (+0.10%)	:arrow_up:

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[SupunS]

Oh I see. So reporting at the beginning of each slab is sufficient, and don't need to report for each storable?

[fxamacker]

Oh I see. So reporting at the beginning of each slab is sufficient, and don't need to report for each storable?

@SupunS Yes. Storables implemented by Cadence are elements in atree (leaf) slabs. Therefore, we don't need to meter storables if we meter slabs.