Add type-driven safe API design exercises

[miguelraz]

There's some cool examples that we can pick up on

• Alex Crichton's Bound and unbound iterator
• The type-safe state machine, and Jon Gjengset's Rust for Rustacean's example
• Builder pattern
• newtype usage

[robamu]

This could be connected to embedded by doing this for a GPIO pin (e.g. Output, Input structures on top of some low-level register access API).