Untrack Cargo.lock (#211)

Tracking Cargo.lock only really makes sense for applications running
straight from this repository (dis, rspirv-autogen) and should usually
represent the set of dependency versions that have been explicitly
tested.  Anyone using these crates in their own projects won't see these
lock versions anyway but get whatever is the latest at the time of
adding or running `cargo update` as per semver.

.gitignore

@@ -1,5 +1,7 @@
-target
 *.swp
 *.pyc
 *.bk
 .DS_Store
+
+/target
+/Cargo.lock