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
 
+# These should only have optional `cc` dependencies which isn't used by deault
+# (and usually works across updates otherwise).
+builtins-shim/Cargo.lock
+builtins-test-intrinsics/Cargo.lock
+
 # Sources for external files
 compiler-rt
 *.tar.gz