You can remove the "std" feature and depend on core no matter what to simplify things. You are basically already doing this because `default-features = ["std"]` does not enable std by default (that would be `default = ["std"]`).
Also you'll want to toss a `#![no_std]` in there or else you're linking std no matter what. Once you do that you can remove the explicit `extern crate core; use core::convert::From` because From is in the core prelude.
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