RFC: OrderedDict

* Includes tests
* Updated OrderedDict, misc. other docs
* Regenerated helpdb.jl

This version is slightly slower than the version at https://github.com/kmsquire/OrderedCollections.jl and discussed in #2548, but thanks to @JeffBezanson pointing out some flaws in my early attempts, it's cleaner and not much slower, and most importantly, it doesn't depend on the internal implementation of `Dict`.

Compared with `Dict`, it has ~40% insertion and ~50% deletion overhead.

I can submit the version at https://github.com/kmsquire/OrderedCollections.jl instead (which is a simple modification of `Dict` in `julia/base`), or make that one available via a package.