Use pointer type instead of value

[morikuni]

Changes

Use pointer type instead of value.

Motivation

Since the error chain become a bigger object, pointer type is better to avoid large data copy.

[codecov-io]

Codecov Report

Merging #22 into master will decrease coverage by 0.31%.
The diff coverage is 95%.

@@            Coverage Diff             @@
##           master      #22      +/-   ##
==========================================
- Coverage   96.05%   95.74%   -0.32%     
==========================================
  Files           5        5              
  Lines         279      282       +3     
==========================================
+ Hits          268      270       +2     
- Misses          7        8       +1     
  Partials        4        4

Impacted Files	Coverage Δ
wrapper.go	99.21% <95%> (-0.79%)	⬇️

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