[Keymap] "Cadet-style" keymap + improved LAYOUT macro for melody96 (#5985)

* [melody96] "Cadet-style" keymap for melody96; LAYOUT macro for hotswap keyboard.

* Rename macro.

* Fix layer types.

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+* The "Cadet-Style" keymap for the melody96.
+* Author: Yonatan Zunger (zunger@gmail.com)
+
+This is an experimental keymap being used both for practical reasons (as my daily driver) and to
+work out the ideas of a "space-cadet-style" keyboard which can type text and mathematical symbols
+with equal ease. It's designed for anyone who frequently needs to do this outside of a LaTeX
+environment, or for anyone who loves the old "Space Cadet" keyboard! And it works at its best when
+you have actual Cadet keycaps (like SA Symbolics) installed, because those keycaps will actually
+show what you get.
+
+The core idea of this keyboard is that, in addition to a QWERTY base layer and a function layer, it
+supports two additional base layers -- the GREEK layer (the analogue of the Space Cadet "Greek," or
+"Front," keys) and the CADET layer (the analogue of the Space Cadet "Top" keys). These layers use
+Unicode to generate all of the mathematical symbols you can find on a traditional Space Cadet
+layout, plus a bunch of extras.
+
+Because Unicode can't be encoded using the traditional USB HID protocol, QMK does some clever but
+horrifying things to fool your OS. One consequence of this is that the shift key needs to be handled
+by the keyboard firmware, not the host OS. To handle this, we have two additional layers --
+SHIFTGREEK and SHIFTCADET -- and handle the flipping between all of these layers here in the
+firmware.
+
+*The simple bit: Using this layout on a Melody96*
+
+At the core of this layout are three special modifier keys and two special lock keys:
+
+* The GREEK key, to the right of the spacebar, activates the GREEK layer. GREEK+SHIFT activates
+  SHIFTGREEK. These keys generate Greek letters on the letter keys (thus the name), and a few
+  mathematical symbols on other keys. They correspond to the notations on the front of traditional
+  Space Cadet keys; if your capset doesn't include those (alas, most don't), they're the "pretty
+  obvious" mappings.
+* The CADET key, to the right of GREEK, activates the CADET layer. These are the symbols above the
+  letters on a Space Cadet layout. CADET+SHIFT activates the SHIFTCADET layer, with even more
+  symbols.
+* The FUNCTION key, to the right of CADET, activates the function layer. This is where you have a
+  reset mechanism, a selector for which Unicode input type you want, and so on.
+
+Additionally, GREEK+ALT is equivalent to CADET. This is handy for other keyboards where you don't
+have room for this many modifiers.
+
+The lock keys are:
+
+* Caps lock, if you use it, will also act as a "shift lock" for the Greek and Cadet layers. Shift
+  lock is slightly different, in that while it is engaged shift will _dis_engage it; that's actually
+  pretty useful when typing math.
+* An additional "layer lock" key, by default where "num lock" usually goes, will lock the choice of
+  base layer. To use it, hold down any invocation of the GREEK or CADET layers, or none at all, and
+  hit lock; it will then put you in that layer. The corresponding modifier key will then toggle you
+  back to the QWERTY layer. (So for example, if you hit GREEK+LAYER_LOCK and release them, you're
+  now typing in Greek; the GREEK modifier would cause you to type QWERTY momentarily. To go back to
+  ordinary QWERTY mode, you'd just hit LAYER_LOCK again with no modifiers held)
+
+To see the full layout, check out the big comment in keymap.c.
+
+*A less-simple bit: Adapting this to other keyboards*
+
+This is really a canary for generic Cadet implementations. Before this can be made generic, a few
+things will have to happen:
+
+(1) Instead of a fixed keymap, this has to be refactored into some kind of array showing the
+mappings of QWERTY-layer keys onto the appropriate code points in the GREEK and CADET layers, and
+some preprocessor magic needs to auto-transform this plus a traditional keymap for the QWERTY layer
+into keymaps for all five of the core layers. (Function layers would presumably be handled on a
+per-keyboard basis)
+
+(2) The standard mapping of those should have some #define's to control things like whether there
+are physical F-keys (you would probably want to move superscript and subscripts onto the numbers if
+there weren't, and figure out what to do with the non-numeric super/sub keys), whether you actually
+want to enable GREEK+ALT=CADET, and so on;
+
+(3) There should be support for controlling indicator LEDs based on the base layer selection,
+caps/shift lock state, and layer lock state, as well as for triggering audio on transitions;
+
+(4) All the core fancy logic in process_record_user which implements the layer handling should be
+factored out into its own function, so that keyboards can easily reuse that, too.
+
+This is a lovely TODO for future work, and could be particularly fun to go along with new releases
+of SA Symbolics and the like. Anyone interested in such things, ping me!