Date handling, new MicroGui widget

[peterhinch]

I developed these for a project of my own, but they are designed to be of general use:

1. A DS3231 driver. The chip is a very cheap and highly accurate RTC. The driver supports alarms and is much more compact than other solutions (the test script is longer than the driver).
2. A Date class The official datetime class is more full-featured but it is quite large. This "micro" solution has no concept of time but enables date calculations and calendar support.
3. A grid widget for MicroGui (image below). This supports spreadsheet-like displays of arbitrary data.

[beetlegigg]

Nice one @peterhinch, I think that will go nicely on one of my ili9341 screens that I connect to a rpi pico, especially as I've been 3d printing some nice cases for that setup that even my wife approves of. I will see if I can also squeeze your label widget onto the same screen for a clock display too. Marvellous.