[Keymap] New kbd67/hotswap keymap for writing both code and math (#4933)

* Custom keymap.

* Fix magic layer, enable Unicode.

* Update README.

* Make Unicode config change keymap-local.

* Move magic to AltGr, simplifying layout.

* Changes per Drashna.

* Fix magic+esc = backtick!

* Change reset implementation per Drashna's suggestion.

* Add a few more symbols so long as the commit is open - √ℜℂℤ⊙ℚℕ and the like.

* Typo fix.

* Slight simplification, drop an extra variable.

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+* Custom keymap for kbd67, for those who need to both code and type math.
+* Author: Yonatan Zunger (zunger@gmail.com)
+
+This keymap is presently rather Mac-centric, as it uses Mac media keys in its function layer and the
+OS X Unicode input mode for the "magic" layers.
+
+*Layer 0 (Base QWERTY):* The layout has a few minor quirks: grave escape, Home/End/PgUp/PgDn in the
+right-hand column (rather than the more common Home/PgUp/PgDn/End), and a pause key between
+backspace and home -- this being something I use for screen lock, a necessity when one works with
+and sits next to security engineers. (Using a heavier switch on the pause key greatly reduces the
+risk of accidental triggering) In OS X style, alt and win are swapped on the left-hand side.
+
+The right-hand alt and gui keys take on a different meaning: right-alt lives up to its old name of
+AltGr by invoking Greek (the "magic" layers, 2 and 3), and right-gui invokes the function layer.
+
+*Layer 1 (Functions):* This layer deliberately sets KC_NO for the keys it doesn't use. I realize
+this isn't common, but I like to keep my functions quite distinct from all other layers.
+
+RESET can be accessed at Fn+Left Shift+Escape.
+
+*Layers 2 and 3*: These are invoked by the "magic" key (AltGr) and by magic+shift. The
+letters mostly produce Greek; the numbers, superscripts (magic) and subscripts (magic+shift); and
+the various other keys, mathematical symbols. There unfortunately aren't nearly enough keys for all
+the symbols I'd like, so magic+alt and magic+alt+shift may end up existing as well, giving this a
+proper Space Cadet effect; but that said, it's going to be hard to remember where all these symbols
+are without some extremely custom keycaps. In the limit of arbitrarily complex layers, the keyboard
+will probably start running LISP and turn into EMACS.