AppendList: An append-only list data structure

This list lets you add stuff to it, even if you're holding a reference to some of its elements.

When should you use it?

• You need to insert into a list while its other elements are borrowed
• You have so much data that Vec reallocations are significant
• Only a single line of unsafe code

When shouldn't you use it?

• You are storing indices into a list, rather than actual references
• Vec reallocations don't matter very much (normally the case!)

What are some features?

• A push(&self, item: T) method (you'd expect push(&mut self, item: T))
• Non-amortized constant-time insertions and indexes
• You can hold onto references in the list