Add Cargo.lock to version control

Historically the official recommendation was to version control
Cargo.lock for applications but gitignore it for libraries.

However, it was later recognized that this is a mistake, and there
are advantages to always version control Cargo.lock, even for libraries:
https://blog.rust-lang.org/2023/08/29/committing-lockfiles/

As a specific example, not having Cargo.lock under version control
caused me a minor annoyance when regenerating expected trybuild output
(b6a3b08)
The file I generated locally didn't match what was happening on GitHub
Actions runner, because my Cargo.lock was different.

Author: Vlad-Shcherbina

.gitignore

@@ -3,10 +3,6 @@
 target/
 test-target/
 
-# Remove Cargo.lock from gitignore if creating an executable, leave it for libraries
-# More information here https://doc.rust-lang.org/cargo/guide/cargo-toml-vs-cargo-lock.html
-Cargo.lock
-
 # These are backup files generated by rustfmt
 **/*.rs.bk