I was thinking about its implementation, too, and ended up deciding that it was probably a performance optimization.

`Vec<T>` stores all data on the heap, so getting anything out of it involves a pointer deref and possibly also an array bounds check. This `NonEmpty<T>` type keeps the first element of the list in a location that supports some low-level optimization that might make a significant difference in situations where accessing the first element is much more common than accessing subsequent elements.