Do something about std::deque

[catamorphism]

It's not used anywhere (it was imported in resolve, but the import wasn't needed) and a comment at the top says "untested... Likely buggy." Though there actually are some tests within the module, I'm not sure if it should be in std if it's been sitting unused for a long time?

In any case, if it does get kept, there's a FIXME before deque::create: "eventually, a proper datatype plus an exported impl would be preferrable" -- that should be addressed.

[eholk]

A deque seems like something that should definitely be in any standard library, so I'd vote for keeping and fixing this one. Unfortunately, it doesn't work at the moment (#2383)