SmallVector is long gone, as it's been first replaced
by OneVector in commit e5e6375352636360add297c1f5a1f37ce71506e9,
which then has been removed entirely in favour of SmallVec in
commit 130a32fa7259d348dc3a684b38e688da398c30bb.
These should make using `ThinVec` feel much more like using `Vec`.
They will allow users of `Vec` to switch to `ThinVec` while continuing
to use `collect()`, `for` loops, and other parts of the iterator API.
I don't know if there were use cases before for using the iterator API
with `ThinVec`, but I would like to start using `ThinVec` in rustdoc,
and having it conform to the iterator API would make the transition
*a lot* easier.
I added a `FromIterator` impl, an `IntoIterator` impl that yields owned
elements, and `IntoIterator` impls that yield immutable or mutable
references to elements. I also added some unit tests for `ThinVec`.
