Typed Transients patch

This patch is not rebased properly onto master. I suggest

git fetch origin master
git checkout ctyp-261 ;; or your branch
git rebase master

Please submit a pull request to typedclojure/core.typed for easier review.
I will ask you to submit a patch once I'm satisfied.

The first line of the commit should not wrap, I think you have about 60 characters
to play with. "Added basic typechecking support for transients" is fine.

Then add a paragraph talking about some of the decisions.

• why are transients covariant

• what do we assume about transient usage (eg. it's not safe to consider a transient
  covariant if it's bashed in place). Outline cases that are ill-typed if transients
  are covariant

eg.

I'm actually unsure if covariance is the best idea if we don't have uniqueness. But
it would help decide if you motivate covariance.

Merged.