Add an example of scrolling text

[mattheww]

The implementation is in a separate microbit-text crate as discussed in #78.

I've tested on micro:bit v1 only.

I did the example using RTIC, but I can just as easily do it without (or both ways).

Does this look sensible? If everyone's happy with microbit-text as it stands I can make it v1.0.0.

[cathiele]

Tested it on a micro:bit v2 and it works (Scrolling once "Hello, world!") but a bit fast i think.

[robyoung]

It looks great, thanks!

I'm a little torn about what to do with the MSRV. We have previously talked about it being informational only so we shouldn't hesitate to bump it (although I can't find that discussion now). However, it feels odd to bump the MSRV just because an example needs it. I can think of a couple of routes forward:

1. Bump the MSRV here and add a comment to the MSRV to be explicit that it includes the examples
2. Exclude building examples from the MSRV build (I'd be happy to do this in a separate PR)
3. Get rid of MSRV all together. The value is debatable when the wider rust ecosystem is moving so quickly.

What do you think? I think my preference is for option 2.

@therealprof do you have an opinion?

[mattheww]

I think the only MSRV problem is that I used edition 2021 for microbit-text.

But it doesn't gain anything from that, so I might as well change it to 2018 and keep the MSRV in sync with microbit's.

[robyoung]

Hah, nice. Well that would neatly sidestep the issue.

[robyoung]

bors r+

[bors[bot]]

Build succeeded:

• build (MSRV)
• build (stable)
• Rustfmt