Proposal: StackedRNNCell

Hi!

I've been using flax for a couple days now and I've been enjoying learning the framework.
In my hacking, I made a stacked rnn cell that basically takes any cell (lstm, gru) and adds vectorized layers plus some niceties like projections and skip connections. https://github.com/mikesol/jaxfun/blob/80629e069641b62352b578e4d7f25e9eac186087/model.py#L151

Would folks be interested in me getting this into PR shape? As I'm brand new to the framework, I'd need help from a more experienced dev to get it over the line, so if someone's willing to put in the time to shepherd the PR, I'd be happy to put it together.