COPYRIGHT: Provide a better explanation of Rust copyrights
COPYRIGHT: Provide a better explanation of Rust copyrights

Avoid implying that any copyrights have been assigned to a separate
entity (such as "The Rust Project Developers") Rust contributors retain
their copyrights, and do not assign them to anyone by contributing.

Remove the inaccurate notice, and provide a clear explanation. Avoid
stating that all files contain copyright notices and/or license notices,
and especially avoid suggesting that the license terms only apply to
files marked as such.

In the process, this also drops a separate notice that implies only some
copyrights are retained by contributors (suggesting that others are
not).

GBGamer/rust,GBGamer/rust,GBGamer/rust,GBGamer/rust,GBGamer/rust,aidancully/rust,graydon/rust,GBGamer/rust,graydon/rust,graydon/rust,aidancully/rust,GBGamer/rust,aidancully/rust,aidancully/rust,graydon/rust,graydon/rust,GBGamer/rust,aidancully/rust,graydon/rust,aidancully/rust