Prevent parsing non-ordered containers
Prevent parsing non-ordered containers

This comes from the discussion of this pull request:

https://github.com/GaloisInc/cereal/pull/20

It was pointed out that fromList on the containers will do the same check
internally that the pull request implements, so it seemed sensible to just
use that instead of replicate its functionality here.

triplepointfive/cereal,triplepointfive/cereal,fpco/fpco-cereal,fpco/fpco-cereal,glguy/cereal,glguy/cereal