Common traits PartialEq, Eq and Hash

[dns2utf8]

I implemented PartialEq, Eq and Hash. Others like Copy or PartialOrd did not seem right.

[dns2utf8]

Currently the build fails prior to rustc 1.12.0 because of the assert_ne! macro.
The DefaultHasher requires rustc 1.13.0.

Should we work around it or update the min version? Debian 9 stable currently serves rust 1.14.0 with the default installation.

[frewsxcv]

PartialEq and Eq make a bit of sense to me. Not sure why you'd need to compare threadpools in the real world, but sure, I think this is fine. I'm a little confused by Hash though. Are you aware of any real world examples involving Hash? Just trying to better understand why the trait implementation is being added

[dns2utf8]

My intention was to implement everything I thought reasonable.
I made a test case with the hash set. I agree do not see an imminent real-world use-case for the hash trait yet.

I implemented it from the list of this document referenced here:
Types eagerly implement common traits ([C-COMMON-TRAITS])
- Copy, Clone, Eq, PartialEq, Ord, PartialOrd, Hash, Debug, Display, Default

We could also split this PR and only merge the PartialEq and Eq.

[frewsxcv]

cherry-picked your PartialEq/Eq impls from this PR and merged them in #81. thanks!

[dns2utf8]

Cool, thank you!