Support for getters

[taras]

I tried creating a getter on a Microstate class and I found that they only work on the hash that's passed to extend. That value is cached and not re-computed for new microstate instances. I would expect getters to be transferred to each new instance.

I created a branch with some failing tests.

I'd like to implement this change. I want to make sure that I'm heading in the right direction. Is the problem that value is cached here?