Test a full no-std build in the no-std-check crate in CI

Rust is incredibly forgiving in attempts to access `std`, making it
rather difficult to test `no-std` properly. In practice, the only
decent way to do so is to actually build for a platform that does
not have `std`, which we do here by building the `no-std-check`
crate for `arm-thumbv7m-none-eabi`.

Author: TheBlueMatt

.github/workflows/build.yml

@@ -128,6 +128,9 @@ jobs:
           # check no-std compatibility across dependencies
           cd no-std-check
           cargo check --verbose --color always
+          rustup target add thumbv7m-none-eabi
+          apt-get -y install gcc-arm-none-eabi
+          cargo build --target=thumbv7m-none-eabi
           cd ..
       - name: Test on no-std builds Rust ${{ matrix.toolchain }} and full code-linking for coverage generation
         if: "matrix.build-no-std && matrix.coverage"

no-std-check/src/lib.rs

@@ -0,0 +1 @@
+#![no_std]