Add HZippable for zipping two HLists

[mwillsey]

Hello! Cool crate you've got here.

I found myself needing to zip HLists, so I implemented it. This is pretty much the easiest, simplest version of HList zipping out there. You could imagine making it more general in a couple ways:

• Make it work over HLists of HLists!
  So zip(h1, h2) == hlist![ hlist![h1.head, h2.head], ...]
• Maybe make it iterable instead of materializing the whole zipped HList, since zips are frequently only used to build up intermediate state, and are typically followed by map or fold. Although, there's a solid chance the optimizer can see through this.
• This version only supports zipping lists of equal length. That seems right for the simplest version, but perhaps you could possibly want more flexibility?

This implements the trait, the inherent method, and a couple simple tests.

Comments welcome. Enjoy!

[lloydmeta]

This is awesome. Definitely fills a missing gap :) thanks!