Lack of dual-licensing

[cppforliving]

Reference https://rust-lang.github.io/api-guidelines/necessities.html#c-permissive

Crates that desire perfect license compatibility with Rust are not recommended to choose only the Apache license. The Apache license, though it is a permissive license, imposes restrictions beyond the MIT and BSD licenses that can discourage or prevent their use in some scenarios, so Apache-only software cannot be used in some situations where most of the Rust runtime stack can.

[marmistrz]

Thanks for the comment. Since Rust is dual-licensed using MIT+Apache, it's makes sense to dual-license the code copy-pasted from the Rust libstd. I wasn't aware of this nitpick.

I'll try to address it within the next week or two, but feel free to submit a PR if you want dual-licensing more quickly :)