Test no_std builds propperly to catch build error regressions in the CI

[CPerezz]

We currently don't check that indeed the crate builds in no_std without pulling any std stuff inside.

To do so, the only way we have is to basically use the following trick:

Nemo157 on Reddit noted that another way to achieve this goal is to build for an ARM target, since std isn't available for >embedded ARM.

To install the target:

$ rustup target add thumbv6m-none-eabi
Then build your crate for this target:

$ cargo build --target thumbv6m-none-eabi
This will fail with a bunch of error messages if std is required by a dependency.

As noted by Nemo157, "that becomes more complicated once you introduce dev-dependencies and optionally-std crates >though".