fix: std requires alloc

[Oppen]

The Rust standard library already requires a global allocator. This means all features that only depend on that should be usable when std is enabled, so let's make that implication explicit in our manifest.

Pull Request type

• Bugfix

What is the current behavior?

You need to explicitly enable alloc even if you're building with std.

What is the new behavior?

Enabling std always enables alloc.

Does this introduce a breaking change?

Technically, yes. You can no longer trigger a build with std but no global allocator. In practice, no, those builds were already broken.