no_std crates should not permit non-no_std dependencies

I couldn't find this exact problem mentioned elsewhere.

Problem

Cargo rightly produces errors for uses of std within a crate but will not complain about dependencies that rely on std when building for platforms that support std. This means that an implicit std requirement can quietly sneak into a project. It seems rather bizarre that cargo does not catch this issue.

Possible Solution(s)

• Make cargo search for uses of std when building dependencies and produce an error
• Do not permit std as a dependency in the dependency graph for no_std crates
• Only permit dependencies that are explicitly tagged with #![no_std] for no_std crates

No files, tests, or entry points are named. Start by tracing Cargo's dependency-resolution and build behavior for no_std crates, then define which proposed restriction should apply and how a failing dependency graph would demonstrate that the work is complete.