Add state

[elefthei]

I needed a state transformer and I copied the one from ITrees with some minimum changes, instance of MonadChoice is from @Grain 's example/Yield/Lang.v.

[YaZko]

Looks good, we can definitely merge this in for now.

On the medium run, I'd like us to think of:

• Adopting the ternary subevent class we developed with @Zdancewic and @euisuny
• Whether we are bold enough to also jump straight into the higher order interpreters
• Whether we need to duplicate the itree interpreters or can factor out things to share them.