Update nft to use StorageNMap

[zjb0807]

Closes: #504

[codecov-commenter]

Codecov Report

Merging #562 (da96af6) into master (81511f1) will increase coverage by 0.01%.
The diff coverage is 100.00%.

@@            Coverage Diff             @@
##           master     #562      +/-   ##
==========================================
+ Coverage   72.58%   72.60%   +0.01%     
==========================================
  Files          76       76              
  Lines        5425     5425              
==========================================
+ Hits         3938     3939       +1     
+ Misses       1487     1486       -1     

Impacted Files	Coverage Δ
nft/src/lib.rs	84.61% <100.00%> (ø)
authority/src/mock.rs	74.46% <0.00%> (+2.12%)	⬆️

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