add GhostCell proof by Joshua Yanovski

_CoqProject

@@ -70,6 +70,7 @@ theories/typing/lib/arc.v
 theories/typing/lib/swap.v
 theories/typing/lib/diverging_static.v
 theories/typing/lib/brandedvec.v
+theories/typing/lib/ghostcell.v
 theories/typing/lib/mutex/mutex.v
 theories/typing/lib/mutex/mutexguard.v
 theories/typing/lib/refcell/refcell.v