Fix licensing on -derive repo

[rozbb]

The curve25591-dalek-derive repo is just a vendored copy of unsafe_target_feature (see #531). As such I have copied over those license files.

[rozbb]

@koute it appears that there is no copyright line on either of these files (here or in your repo). Would you like to add a copyright line crediting yourself? Let me know what to write.

[koute]

@koute it appears that there is no copyright line on either of these files (here or in your repo). Would you like to add a copyright line crediting yourself? Let me know what to write.

I usually don't add explicit copyright lines since legally it makes no difference and practically it's mostly useless (unless you're going to add every contributor, in which case it can act as a list of everyone who has copyright over a given work, although that's kinda redundant if you have the git history) and can sometimes be a bad idea, but if you do want to add an explicit copyright line feel free to use the name of my employer: Parity Technologies (UK) Ltd.

[rozbb]

Understood, thank you for the quick response. I have no strong desire to include a copyright line, so we can just leave it how it is.

[rozbb]

Quick note: the build warnings I was getting were becausefield.rs had unused pub use statements, but the module itself is only pub(crate). They're vestigial, it seems. Now the module is all pub(crate)