Add experimental flag for bind struct

[sagikazarmark]

Fixes #1851
Fixes #1721

[github-actions]

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[andig]

Love it- much simpler to use than the compile switches. Doesn't this also replace #1715? Wondering why that's not visible in the diff? However, it's not possible to use with the global viper instance, right?

[sagikazarmark]

The build tag didn't go away. You can still set it which means it's going to be enabled by default for every Viper instance. Or you can enable it on a case-by-case basis using the option.

[haoming29]

Is there a way to enable this feature with the global instance other than doing the build flag? Thanks!
UPDATE: nvm, found in #1856