Generate a SetDefaults function

[ptodev]

A SetDefaults() function would set the structure fields to their default values. It could be particularly useful for users of mapstructure.

We could also have a Validate() function? I could try to add it if we agree on how a SetDefaults() could be implemented, since I suppose the implementation would be similar.

This branch doesn't depend on subschemas, but I set the merge target to omissis:feature/implement-subschemas because I've been using subschemas for another project, and my changes happened to depend on refactoring being done on the subschema branch :)

[omissis]

This is an interesting idea, and it will probably benefit from #293's work as well. given the dependency on the subschemas branch, I would probably try to focus on merging that one first, and then come back here.
Unfortunately that branch is a bit behind main now, and I have been reluctant to merge that work because I want to take the time to test it properly. Is there any chance you can help with that? Even just providing some schemas I can base my tests on (if they are OSS) would do, thanks!