Refine type for MAC

[yaahc]

part of #123

[codecov]

Codecov Report

Merging #577 into main will increase coverage by 2.59%.
The diff coverage is 60.39%.

@@            Coverage Diff             @@
##             main     #577      +/-   ##
==========================================
+ Coverage   53.05%   55.65%   +2.59%     
==========================================
  Files          68       88      +20     
  Lines        3212     4088     +876     
==========================================
+ Hits         1704     2275     +571     
- Misses       1508     1813     +305     

Impacted Files	Coverage Δ
tower-batch/src/error.rs	0.00% <0.00%> (ø)
tower-batch/src/layer.rs	0.00% <0.00%> (ø)
zebra-chain/src/block.rs	87.50% <ø> (+16.53%)	⬆️
zebra-chain/src/keys/sapling.rs	85.30% <ø> (ø)
zebra-chain/src/lib.rs	50.00% <ø> (ø)
zebra-chain/src/merkle_tree.rs	0.00% <ø> (ø)
zebra-chain/src/note_commitment_tree.rs	0.00% <ø> (ø)
zebra-chain/src/serde_helpers.rs	0.00% <0.00%> (ø)
zebra-chain/src/transaction.rs	16.66% <ø> (+16.66%)	⬆️
zebra-chain/src/transaction/joinsplit.rs	100.00% <ø> (+4.91%)	⬆️
... and 90 more

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[dconnolly]

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