der: v0.6 release tracking issue

[tarcieri]

New versions of all of the der crate's dependencies are now released, which means it's possible to do another release of der/der_derive.

Ideally I think we can get any prospective breaking changes into the forthcoming release so the version in git can stay v0.6 as long as possible as opposed to having to immediately bump to v0.7.0-pre.

Here's a list of changes that would be nice to get into a v0.6 release of the der crate:

• der: support for the REAL type #304
• der: Reader and Writer traits #525
• der: add EncodeRef/EncodeValueRef; remove OptionalRef #604
• der: Document and SecretDocument types #571
• der: remove decode-time SET OF ordering checks #625

[tarcieri]

All of the changes have now been implemented.

I am going to cut another round of prereleases, and if everything looks good cut a final v0.6 release soon.

[roblabla]

The 0.6 version has been cut, I suppose this can be closed?

[tarcieri]

Yep, thanks!