Implement “internally create a new object implementing the interface”

[ExE-Boss]

Extracted from #213 because of #213 (comment):

The generated snapshots here seem overcomplicated, and it's not clear why you're modifying existing functionality (like new()).

Blocks:

• Implement [LegacyFactoryFunction] #213

[domenic]

Looks good after rebase

[domenic]

I'm sad about the extra size for such an edge-case feature, but it seems unavoidable, and I appreciate the work you did to make it shorter by using different steps than the spec (with equivalent logic).