change drain to be consistent with Vec

[lcnr]

drain has a different signature than Vec, in case this is something that is worth fixing, I would be glad to post a PR.

SmallVec: pub fn drain(&mut self) -> Drain<A::Item>
Vec: pub fn drain<R>(&mut self, range: R) -> Drain<T> where R: RangeBounds<usize>

[mbrubeck]

This will only work on Rust 1.28 and later, because RangeBounds was not stable in earlier versions. (The current version of smallvec is compatible with Rust 1.20.)

I think we should release a new major version of smallvec soon that increases the Rust version requirement, along with making some of the other breaking changes listed in #73. If you'd like to submit a PR for this issue, we can include it in that release.

[lcnr]

This seems far more complicated than expected, as 0..start might be smaller than inline_size which causes problems as the union is suddenly getting misinterpreted. While I think that I have a safe solution, it will take a few more days until it's finished.

Edit: never mind, I have misunderstood how the union feature works.