Would there be any issues with / objections to having `define-key` return the keymap it adds a key to? It currently returns `nil`, making it a statement (bleh!) rather than a function.
If `define-key` returned the keymap, calls to it could be chained together in convenient ways, e.g.
```
(thread-first (make-sparse-keymap)
(define-key (kbd "a") #'foo)
(define-key (kbd "b") #'bar))
```