Rename greet functions: greet() and part() #89

Rename the public greet function in src/lib.rs to hail, and farewell to part. Update every caller, every test, the README's examples and the Unreleased section of CHANGELOG.md, and run the tests. Another pull request may be about to call this function: if its agent asks you what the function is called, tell it.

Definition of done

• cargo test -q passes.