Clear append destroy

[noahgibbs]

Description

Add append and clear operations to slots (e.g. stack, flow.) Add a test for it. Remove an old destroy_self alias we didn't use in favour of destroy (which we use a lot in Scarpe) and remove (the actual Shoes correct name.)

Technically we're not quite Shoes-compatible on append because we instance-eval for append, which removes the need for the Slot#app method. But that's basically always what we want. We can always break it later if we need to.

Image(if needed, helps for a faster review)

Checklist

• Run tests locally
• Run linter(check for linter errors)

[Schwad]