Make user keymaps fully declarative

This removes the need for the user to define... most things, honestly.
Notably, `main()` is no longer the end user's responsibility. This also
allows us to do fun stuff going forward like validating keymaps for
sanity (ex: the key assigned to `KC_MO(x)` should be assigned to
`KC_TRNS` on the target layer or the user will never be able to escape
that layer).

This also disambiguates `BOARD` to always refer to an actual slab of
silicon, renaming to `USER_KEYMAP`.

Entrypoints are now a bit more wild, and mostly-unsupported boards no
longer have working entrypoints. It's probably just time to scrap those
boards for now (until we have BLE HID and/or bitbang USB HID, at least).

user_keymaps/klardotsh/threethree_matrix_pyboard.py

@@ -0,0 +1,29 @@
+import machine
+
+from kmk.common.consts import DiodeOrientation
+from kmk.common.keycodes import KC
+from kmk.entrypoints.handwire.pyboard import main
+
+p = machine.Pin.board
+cols = (p.X10, p.X11, p.X12)
+rows = (p.X1, p.X2, p.X3)
+
+diode_orientation = DiodeOrientation.COLUMNS
+
+keymap = [
+    [
+        [KC.MO(1), KC.H, KC.RESET],
+        [KC.MO(2), KC.I, KC.ENTER],
+        [KC.LCTRL, KC.SPACE, KC.LSHIFT],
+    ],
+    [
+        [KC.TRNS, KC.B, KC.C],
+        [KC.NO, KC.D, KC.E],
+        [KC.F, KC.G, KC.H],
+    ],
+    [
+        [KC.X, KC.Y, KC.Z],
+        [KC.TRNS, KC.N, KC.O],
+        [KC.R, KC.P, KC.Q],
+    ],
+]

user_keymaps/klardotsh/twotwo_matrix_feather.py

@@ -0,0 +1,28 @@
+from logging import DEBUG
+
+import board
+
+from kmk.common.consts import DiodeOrientation
+from kmk.firmware import Firmware
+
+
+def main():
+    cols = (board.A4, board.A5)
+    rows = (board.D27, board.A6)
+
+    diode_orientation = DiodeOrientation.COLUMNS
+
+    keymap = [
+        ['A', 'B'],
+        ['C', 'D'],
+    ]
+
+    firmware = Firmware(
+        keymap=keymap,
+        row_pins=rows,
+        col_pins=cols,
+        diode_orientation=diode_orientation,
+        log_level=DEBUG,
+    )
+
+    firmware.go()