change 'cont:' syntax to have explicit binder

theories/lang/notation.v

theories/typing/cont.v

theories/typing/lib/option.v

theories/typing/lib/rc.v

theories/typing/lib/refcell/ref_code.v

theories/typing/lib/refcell/refcell_code.v

theories/typing/lib/refcell/refmut_code.v

theories/typing/lib/rwlock/rwlock_code.v

theories/typing/lib/rwlock/rwlockreadguard_code.v