fix(gnovm): allow private struct fields to be set

[mvertes]

Go reflect doesn't allow to set unexported struct fields. We use unsafe to override this limitation.

Fixes #1155 and #3802

[Gno2D2]

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[ltzmaxwell]

I'm considering the side effects, this approach seems appropriate cuz it's limited for the given situation: specifically for the gonative call scenario, is it?. so it looks good to me.
cc: @thehowl .

[thehowl]

Not ideal; with the removal of NativeValue/NativeType I want to get to having fmt.Printf in a native implementation that works on TypeValues (#1361) rather than with reflect. But this does the job for now, and solves a hiccup people have repeatedly; so let's merge this.