Add Cargo.lock.

rust-cuda is a library. Is committing its `Cargo.lock` a good idea?
- https://blog.rust-lang.org/2023/08/29/committing-lockfiles/ says
  "maybe": it depends on the details of the library.
- https://rust-lang.github.io/rustup/overrides.html#the-toolchain-file
  says "Cargo.lock should probably be tracked too if the toolchain is
  pinned to a specific release, to avoid potential compatibility issues
  with dependencies." (rust-cuda pins to a specific Nightly release.)
- rust-gpu has a committed `Cargo.lock`.

Overall, the signs point to "yes".

Author: nnethercote

.gitignore

@@ -1,5 +1,4 @@
 book
 /target
-Cargo.lock
 **/.vscode
-.devcontainer
+.devcontainer