Continue to shuffle and burn stuff

- Remove the concept of "mcus". With only one target platform
(CircuitPython), it no longer makes a bunch of sense and has been kept
around for "what if" reasons, complicating our import chains and eating
up RAM for pointless subclasses. If you're a `board`, you derive from
`KeyboardConfig`. If you're a handwire, the user will derive from
`KeyboardConfig`. The end. As part of this, `kmk.hid` was refactored
heavily to emphasize that CircuitPython is our only supported HID stack,
with stubs for future HID implementations (`USB_HID` becomes
`AbstractHID`, probably only usable for testing purposes,
`CircuitPython_USB_HID` becomes `USBHID`, and `BLEHID` is added with an
immediate `NotImplementedError` on instantiation)

- `KeyboardConfig` can now take a HID type at runtime. The NRF52840
boards will happily run in either configuration once CircuitPython
support is in place, and a completely separate `mcu` subclass for each
mode made no sense. This also potentially allows runtime *swaps* of HID
driver down the line, but no code has been added to this effect. The
default, and only functional value, for this is `HIDModes.USB`

- Most consts have been moved to more logical homes - often, the main
or, often only, component that uses them. `DiodeOrientation` moved to
`kmk.matrix`, and anything HID-related moved to `kmk.hid`

docs/config_and_keymap.md

@@ -18,9 +18,7 @@ interface between end users and the inner workings of KMK. Let's dive in!
   some day.
 
 - Import the `KeyboardConfig` object for your keyboard from `kmk.boards` (or, if
-  handwiring your keyboard, import `KeyboardConfig` from the appropriate MCU for your
-  board from `kmk.mcus`. See `hardware.md` for the list of supported boards and
-  map this to the correct Python module under either of those paths.
+  handwiring your keyboard, import `KeyboardConfig` from `kmk.keyboard_config`).
 
 - Assign a `KeyboardConfig` instance to a variable (ex. `keyboard = KeyboardConfig()` - note
   the parentheses)
@@ -37,6 +35,8 @@ if __name__ == '__main__':
   for example:
 
 ```python
+from kmk.matrix import DiodeOrientation
+
 col_pins = (P.SCK, P.MOSI, P.MISO, P.RX, P.TX, P.D4)
 row_pins = (P.D10, P.D11, P.D12, P.D13, P.D9, P.D6, P.D5, P.SCL)
 rollover_cols_every_rows = 4