Apply rustfmt's option `group_imports=StdExternalCrate`

[kimhanbeom]

• Apply group_imports=StdExternalCrate format.
• Fix clippy error.

Related issue: #760

[codecov[bot]]

Codecov Report

Attention: Patch coverage is 0% with 3 lines in your changes missing coverage. Please review.

Project coverage is 48.64%. Comparing base (e40b0ee) to head (cef5723). Report is 17 commits behind head on tis.

Files	Patch %	Lines
src/storage.rs	0.00%	3 Missing :warning:

Additional details and impacted files

```diff @@ Coverage Diff @@ ## tis #772 +/- ## ========================================== - Coverage 49.54% 48.64% -0.90% ========================================== Files 29 29 Lines 8296 8375 +79 ========================================== - Hits 4110 4074 -36 - Misses 4186 4301 +115 ```

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