Support marshalling/unmarshalling map interface

[at-wat]

fix #120

[codecov]

Codecov Report

Merging #123 into master will increase coverage by 0.01%.
The diff coverage is 100%.

@@            Coverage Diff             @@
##           master     #123      +/-   ##
==========================================
+ Coverage   99.37%   99.39%   +0.01%     
==========================================
  Files          22       22              
  Lines        1272     1313      +41     
==========================================
+ Hits         1264     1305      +41     
  Misses          4        4              
  Partials        4        4

Impacted Files	Coverage Δ
marshal.go	98.76% <100%> (+0.15%)	⬆️
unmarshal.go	98.68% <100%> (+0.24%)	⬆️
elementtable.go	100% <0%> (ø)	⬆️

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