Add Cargo.lock (again)

`Cargo.lock` files were added in huggingface#229 and then removed about a month
later in huggingface#253 (with the justification that "cargo.lock messes up
benchmarks").

Shortly after, the Rust project guidance was updated to encourage
committing lockfiles:

https://blog.rust-lang.org/2023/08/29/committing-lockfiles.html
https://doc.rust-lang.org/cargo/faq.html#why-have-cargolock-in-version-control
rust-lang/cargo#8728

Let's add the lockfiles back to make builds reproducible and
deterministic, especially for consumers of Python bindings.

.gitignore

@@ -1,6 +1,4 @@
 safetensors/target
-safetensors/**/Cargo.lock
-bindings/python/Cargo.lock
 *.bin
 *.h5
 *.msgpack