MVP closures

[MartinKavik]

• removed global state
• removed seed::update
• updateMap called with entries

Note:
Closure::new isn't in stable Rust - I've written it for Seed and allowed to use in user apps, but it's limited, you can't change its signature (like add parameters). Alternative is Closure::wrap or switch to nightly Rust (Seed is not 100% tested on nigthly, e.g. I had to add some type annotations because of it).

Function start should be refactored a little bit in the future.