Make tests pass on no_std

[laralove143]

Tests didn't pass with cargo test --no-default-features because tests using format! weren't exempt from no_std, this PR adds cfg(feature = "std") where required, and changes uses of format! to to_string where possible (also adding alloc crate for the tests)

[djc]

Thanks! Could you add a CI job to make sure we don't regress this?