fix warnings

theories/lang/lifting.v

theories/lang/tactics.v

theories/typing/base.v

theories/typing/borrow.v

theories/typing/cont_context.v

theories/typing/fixpoint.v

theories/typing/function.v

theories/typing/lft_contexts.v

theories/typing/lib/cell.v

theories/typing/lib/refcell/ref.v

theories/typing/lib/refcell/refcell.v

theories/typing/lib/refcell/refmut.v

theories/typing/lib/rwlock/rwlock.v

theories/typing/lib/rwlock/rwlockreadguard.v

theories/typing/lib/rwlock/rwlockwriteguard.v

theories/typing/own.v

theories/typing/product.v

theories/typing/product_split.v

theories/typing/shr_bor.v

theories/typing/sum.v