add new_with_capacity constructor

[bbstilson]

This PR adds a new constructor new_with_capacity, which mimics the Vec::with_capacity API. I shamelessly stole almost all the documentation from Rust 🤓

Choices made:

• Naming: I started with with_capacity, but because we must instantiate with an element I leaned towards a name that differentiated itself to avoid confusion and landed on new_with_capacity. No strong feelings on this and happy to change.
• capacity refers to the size of the NonEmpty struct as though it were a Vec. This is sort of a weird one, but there are two motivations.
  • First, I didn't want to expose the inner workings of how the data are stored under the hood in NonEmpty. The mental model of NonEmpty is that it's a Vec but always has at least one element in it. Because we require a value when constructing a NonEmpty vec, the size must always be at least 1 even though internally the tail Vec will be empty.
  • Second, requiring capacity to always be at least one matches the existing behavior of NonEmpty::capacity, which returns (roughly) 1 + tail.len().