meta: Add a root Cargo.lock file

In order to improve upon some inconsistencies seen across CI runs and
locally, add a Cargo.lock file for the main workspace.

Author: tgross35

.gitignore

@@ -1,7 +1,11 @@
 # Rust files
-Cargo.lock
 target
 
+# We have a couple of potential Cargo.lock files, but we only the root
+# workspace should have dependencies other than (optional) `cc`.
+Cargo.lock
+!/Cargo.lock
+
 # Sources for external files
 compiler-rt
 *.tar.gz