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).

kmk/entrypoints/handwire/Pipfile

@@ -0,0 +1,11 @@
+[[source]]
+url = "https://pypi.org/simple"
+verify_ssl = true
+name = "pypi"
+
+[packages]
+
+[dev-packages]
+
+[requires]
+python_version = "3.7"