Toggle for auto pickaxe use

[bricks]

'U'sing a pickaxe, setting a hotkey, and targeting are all really tedious when you are just trying to find a diggable wall or check for disconnected parts of a map. A toggle that changed you from "wall bump" (AKA default mode) to "dig mode" (walking into a wall causes you to try to destroy it) would be a welcome addition, even if the keybinding is unbound by default. A message overlay like the caps-lock scroll message would keep it from leading to player deaths.