`moxie::Key` is and isn't a smart pointer

[anp]

Two API guidelines are in conflict for Key:

• Smart pointers do not add inherent methods (C-SMART-PTR)
• Only smart pointers implement Deref and DerefMut (C-DEREF)

Key implements Deref (but not DerefMut), and it adds inherent methods. Its deref impl is useful for sharing the "current state" with app logic as it is.

[anp]

Key shouldn't implement Deref -- it has footguns! Notably it's pretty easy to rely on the deref behavior immediately after having updated the key, only to find it hasn't actually changed.