More robust embedded tests

[nyonson]

I am experimenting with a pretty hack addition to the test subcommand where it detects if no-std is applied to a package, and if so, runs a build for the thumbv7m-none-eabi target. Maybe better than nothing, but I am not sure how valuable.

I want to learn more on the history of some of the embedded directories in the rust-bitcoin projects which have more thorough testing. Perhaps it make sense to codify a pattern along those lines?

[tcharding]

Those crates were from before my time. You plan sounds reasonable.

[apoelstra]

I believe the existing embedded tests were contributed by Elichai (who hasn't been around for some years now) in response to embedded bugs that he was seeing as a downstream user.

They received very little review other than some cursory testing that they seemed to work, and over the years when things would break in them we'd tend to just remove stuff rather than fixing them.

I think it's fine to just get rid of them.