Sometimes you need to change a structure, for example, add a new property to a struct that is saved in user defaults. You can already do this without migrations by making the property optional, but that complicates a lot of code, and it doesn't work for more comprehensive changes.

I don't have a clear idea of how this would work, but I'm thinking the user would register migrations that run as early as possible at startup or on the first Defaults call if before that.

Defaults.migrations = [
	.Migration(version: "1.2.3") {
		// How to handle it here?
	}
]

I'm open to ideas.