Use value copy instead of pointer copy in stateObject.deepCopy

[summerpro]

From：cosmos/ethermint#741

Close: cosmos/ethermint#740

• Targeted PR against correct branch (see CONTRIBUTING.md)

• Linked to github-issue with discussion and accepted design OR link to spec that describes this work.

• Wrote tests

• Updated relevant documentation (docs/)

• Added a relevant changelog entry to the Unreleased section in CHANGELOG.md

• Reviewed Files changed in the github PR explorer

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For Admin Use:

• Added appropriate labels to PR (ex. wip, ready-for-review, docs)
• Reviewers Assigned
• Squashed all commits, uses message "Merge PR #XYZ: [title]" (coding standards)

[codecov-io]

Codecov Report

Merging #580 (4139022) into dev (c8e9201) will decrease coverage by 0.00%.
The diff coverage is 50.00%.

@@            Coverage Diff             @@
##              dev     #580      +/-   ##
==========================================
- Coverage   66.56%   66.55%   -0.01%     
==========================================
  Files         306      306              
  Lines       19038    19045       +7     
==========================================
+ Hits        12672    12675       +3     
- Misses       5641     5643       +2     
- Partials      725      727       +2     

Impacted Files	Coverage Δ
x/evm/types/state_object.go	81.43% <50.00%> (-1.69%)	⬇️

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