Keyboard Input (3 hours)

If everything is handled correctly, the UI should be calling methods that the rest of the program can access. If this is the case, it should be nothing more than simply calling these when buttons are pressed.

Depending on Rust's stuff, this is *might* be ridiculously easy... it may only take a few minutes to get working.

No files or tests are named. Start by locating the Java Swing UI's button handlers and the existing methods they call; done means keyboard input triggers the same calculator actions as the buttons, verified manually.

Written by the indexing model from the issue text.