support macaddr::MacAddr6 with Pg

[fenollp]

No description provided.

[weiznich]

Thanks for opening this PR. I want to clarify a few point here before you put more work into the implementation.
As this adds another dependency to diesel I want to be sure that this dependency is:

• Maintained
• Trusted by the community
• Commonly used

Additionally I would like to see arguments for whether that impl needs to be in diesel and not in this dependency crate (or another third party crate).