`append_one` is now a method of `Vec`

(Also added a type hint for the compiler to deduce what `flags_out`
contains at the time `append_one` is called.)
(Changed all structs to public vis. by default)
Merge branch 'exp/fix-visibility' into exp/fix-append-one
Merge pull request #65 from drbawb/exp/fix-append-one

Use new `Vec#append_one()` method