Allow mutable exported global. Fix #137

[kateinoigakukun]

Ref: WebAssembly/spec#814

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[kateinoigakukun]

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[NikVolf]

worth mentioning that it closes #137

[taegyunkim]

Hi @kateinoigakukun, are you sill pursuing this? I'm interested in updating testsuites submodule to HEAD, and can help or take from here on.

For green CI, you have to also update a test in validation subcrate, specifically this one c9f0597

[taegyunkim]

@kateinoigakukun I'll accept your thumbs up as a sign for me to take this.

[pepyakin]

This PR was superseded by #228. Thanks for the effort anyway @kateinoigakukun !