Make [lft] an Opaque type, when seen from outside of the lifetime logic model.

theories/lifetime/lifetime.v

theories/lifetime/lifetime_sig.v

theories/lifetime/model/borrow.v

theories/lifetime/model/definitions.v

theories/lifetime/model/primitive.v

theories/lifetime/model/reborrow.v

theories/typing/borrow.v

theories/typing/lft_contexts.v

theories/typing/lib/refcell/ref.v

theories/typing/lib/refcell/ref_code.v

theories/typing/lib/refcell/refcell.v

theories/typing/lib/refcell/refcell_code.v

theories/typing/lib/refcell/refmut.v

theories/typing/lib/refcell/refmut_code.v

theories/typing/lib/rwlock/rwlock.v

theories/typing/lib/rwlock/rwlock_code.v

theories/typing/lib/rwlock/rwlockreadguard.v

theories/typing/lib/rwlock/rwlockwriteguard.v

theories/typing/lib/rwlock/rwlockwriteguard_code.v

theories/typing/programs.v