migrate to mutate_exist

[ferrell-code]

closes #596

[xlc]

Need some new tests to verify the storage is removed

[codecov-commenter]

Codecov Report

Merging #597 (7e6a19a) into master (5712c07) will increase coverage by 0.14%.
The diff coverage is 93.33%.

@@            Coverage Diff             @@
##           master     #597      +/-   ##
==========================================
+ Coverage   75.38%   75.52%   +0.14%     
==========================================
  Files          76       76              
  Lines        5931     5945      +14     
==========================================
+ Hits         4471     4490      +19     
+ Misses       1460     1455       -5     

Impacted Files	Coverage Δ
bencher/src/build_wasm/mod.rs	0.00% <ø> (ø)
bencher/src/build_wasm/wasm_project.rs	0.00% <0.00%> (ø)
bencher/src/handler.rs	0.00% <0.00%> (ø)
nft/src/lib.rs	84.61% <0.00%> (ø)
tokens/src/mock.rs	26.76% <ø> (ø)
xtokens/src/lib.rs	71.91% <0.00%> (ø)
benchmarking/src/lib.rs	30.08% <68.96%> (ø)
rewards/src/lib.rs	82.35% <77.77%> (+0.81%)	⬆️
auction/src/tests.rs	100.00% <100.00%> (ø)
authority/src/mock.rs	73.46% <100.00%> (+1.12%)	⬆️
... and 16 more

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