account: change instance instead of prototype

[libotony]

fix #38

[codecov-io]

Codecov Report

Merging #40 into master will not change coverage.
The diff coverage is 100%.

@@          Coverage Diff          @@
##           master    #40   +/-   ##
=====================================
  Coverage     100%   100%           
=====================================
  Files          13     13           
  Lines         519    518    -1     
  Branches      145    145           
=====================================
- Hits          519    518    -1

Impacted Files	Coverage Δ
src/extend/accounts.ts	100% <100%> (ø)	⬆️

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