`pow(a, b, c)` can compute `(a ** b) % c` efficiently (in time and memory).
This can be useful for extremely specific applications, like implementing
the RSA cryptosystem. For typical uses of CircuitPython, this is not an
important feature. A survey of the bundle and learn system didn't find
any uses.
Disable it on M0 builds so that we can fit in needed upgrades to the USB
stack.
This adds the `async def` and `await` verbs to valid CircuitPython syntax using the Micropython implementation.
Consider:
```
>>> class Awaitable:
... def __iter__(self):
... for i in range(3):
... print('awaiting', i)
... yield
... return 42
...
>>> async def wait_for_it():
... a = Awaitable()
... result = await a
... return result
...
>>> task = wait_for_it()
>>> next(task)
awaiting 0
>>> next(task)
awaiting 1
>>> next(task)
awaiting 2
>>> next(task)
Traceback (most recent call last):
File "<stdin>", line 1, in <module>
StopIteration: 42
>>>
```
and more excitingly:
```
>>> async def it_awaits_a_subtask():
... value = await wait_for_it()
... print('twice as good', value * 2)
...
>>> task = it_awaits_a_subtask()
>>> next(task)
awaiting 0
>>> next(task)
awaiting 1
>>> next(task)
awaiting 2
>>> next(task)
twice as good 84
Traceback (most recent call last):
File "<stdin>", line 1, in <module>
StopIteration:
```
Note that this is just syntax plumbing, not an all-encompassing implementation of an asynchronous task scheduler or asynchronous hardware apis.
uasyncio might be a good module to bring in, or something else - but the standard Python syntax does not _strictly require_ deeper hardware
support.
Micropython implements the await verb via the __iter__ function rather than __await__. It's okay.
The syntax being present will enable users to write clean and expressive multi-step state machines that are written serially and interleaved
according to the rules provided by those users.
Given that this does not include an all-encompassing C scheduler, this is expected to be an advanced functionality until the community settles
on the future of deep hardware support for async/await in CircuitPython. Users will implement yield-based schedulers and tasks wrapping
synchronous hardware APIs with polling to avoid blocking, while their application business logic gets simple `await` statements.
Tested & working:
* Send standard packets
* Receive standard packets (1 FIFO, no filter)
Interoperation between SAM E54 Xplained running this tree and
MicroPython running on STM32F405 Feather with an external
transceiver was also tested.
Many other aspects of a full implementation are not yet present,
such as error detection and recovery.
The font is missing many characters and the build needs the space.
We can optimize font storage when we get a good font.
The serial output will work as usual.
Testing performed: That a card is successfully mounted on Pygamer with
the built in SD card slot
This module is enabled for most FULL_BUILD boards, but is disabled for
samd21 ("M0"), litex, and pca10100 for various reasons.
