Add dynamic heap usage (alloc stabilization)

[johnthagen]

Per rust-lang/rust#59675, alloc will stabilize with Rust 1.36.0. It would be very helpful if the Discovery book was then updated to show how to use a real dynamic Vec when this stabilizes. This takes embedded one important step further in ergonomics.

Cross links

• Embedded Commercial Rust Gaps wg#336
• How can dynamically sized Vec<T> be using on STM32? wg#335