Bump to cortex m 0.7.0

[ithinuel]

This helps reducing the number of crates built.

[ithinuel]

Note the first commit is the tip of the v0.5.1 tag.

[ithinuel]

@jamesmunns is this crate still maintained?

[jamesmunns]

@ithinuel not as well as it should be. I've not actively used it as much as I have in the past, and combined with a couple of difficult architectural items that I want to tackle but haven't found the right approach for, and being busy with other things, I haven't given it the love it needs.

If you specifically are interested in being a maintainer, or helping to keep the lights on, I will gladly add permissions for you to the repo and crates io publishing. Feel free to let me know here or send me an email or matrix DM to discuss.

[ithinuel]

I'm happy to help keep the light on, but I cannot commit for maintaintership as my availability is pretty volatile these days.

Also, I'm closing this PR in favour of #98.