prove writing to Cell; tune some notation

theories/lifetime/frac_borrow.v

theories/lifetime/na_borrow.v

theories/lifetime/shr_borrow.v

theories/typing/own.v

theories/typing/shr_bor.v

theories/typing/tests/get_x.v

theories/typing/tests/init_prod.v

theories/typing/tests/rebor.v

theories/typing/tests/unbox.v

theories/typing/uninit.v

theories/typing/uniq_bor.v

theories/typing/unsafe/cell.v