Key input handler

[byorgey]

Ability to code your own input handler routines. Closes #102 . Fixes #1210 .

• Adds a new type key to represent keypresses
• Adds a primitive function key : text -> key which can handle usual letters, numbers, etc. as well as special keys like "Down" etc, as well as modifier key prefixes like key "A-C-Del". swarm generate keys generates a list of all recognized special key names.
• New command installKeyHandler : text -> (key -> cmd unit) -> cmd unit which sets the "current key handler". The text value is a hint line to display in the secondary key hints menu while the handler is running. The global shortcut M-k toggles the currently installed handler.
• Add a keyboard device to provide these commands, as well as a key entity (the recipe for a keyboard is 16 keys + 1 board).
• Add a few examples in the examples folder.
• Add an installed keyboard to the building-bridges challenge.