Not using unsafe

[shamaton]

The impact on Default(Map) Format was great, so I modified the code to suppress it.
Performance will be less, but it can use safe completely.
issue: #25

[codecov]

Codecov Report

Merging #27 (48efb60) into master (e325f82) will increase coverage by 0.10%.
The diff coverage is 100.00%.

@@            Coverage Diff             @@
##           master      #27      +/-   ##
==========================================
+ Coverage   81.18%   81.29%   +0.10%     
==========================================
  Files          38       38              
  Lines        2402     2416      +14     
==========================================
+ Hits         1950     1964      +14     
  Misses        284      284              
  Partials      168      168              

Impacted Files	Coverage Δ
internal/decoding/decoding.go	76.16% <100.00%> (ø)
internal/decoding/string.go	93.54% <100.00%> (+0.95%)	⬆️
internal/decoding/struct.go	78.10% <100.00%> (+1.72%)	⬆️

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