e019: Let's `Clone` a `Cow`

The final pieces of the story for (single-threaded) memory management in Rust.
Notes
Sometimes, we actually do need to copy types. Wouldn't it be nice if Rust gave us a convenient way to do that when it's convenient, or when the cost is low enough that the ergonomic tradeoffs are worth it? Well, perhaps unsurprisingly, it does! The Copy and Clone traits, plus the Cow type, give us everything we need!
Links

underhanded.rs
The typess

std::marker::Copy

"Copy types" in the book
"Stack-Only Data: Copy" in the new book
7.2.0.2 Moved and copied types:

When a local variable is used as an rvalue, the variable will be copied if its type implements Copy. All others are moved.

Extended example in "Traits" section of new book

std::clone::Clone
std::borrow::Cow

Default implementations

discussion in the current book
discussion in the new book

Supertraits

from the discussion in the reference (6.1.9 Traits):

Traits may inherit from other traits.... The syntax