mapslice-json issues

Cannot parse an object in an array or an object in an array in an object

mapslice-json cannot parse below JSON data. ```json {"a":[{"s":"test","i":3,"b":true},{"b":false,"f":3.14}]} ```

fix DATA RACE when it calls nextIndex()

I found a DATA RACE error when I wrote some test code using mapslice-json. You can reproduce quickly by invoking a test function included in this PR. ``` $ go...

t2y

Bump golang.org/x/sys from 0.0.0-20210630005230-0f9fa26af87c to 0.1.0

Bumps [golang.org/x/sys](https://github.com/golang/sys) from 0.0.0-20210630005230-0f9fa26af87c to 0.1.0. Commits See full diff in compare view [![Dependabot compatibility score](https://dependabot-badges.githubapp.com/badges/compatibility_score?dependency-name=golang.org/x/sys&package-manager=go_modules&previous-version=0.0.0-20210630005230-0f9fa26af87c&new-version=0.1.0)](https://docs.github.com/en/github/managing-security-vulnerabilities/about-dependabot-security-updates#about-compatibility-scores) Dependabot will resolve any conflicts with this PR as long as you don't alter...

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