Adjustable runtime Parameters

I was working on trying to get a nice looking [fibonacci tree][1], and I had two constants
```
branchAngle = 30
splayAngle = 30
```
which I keep tweaking, but it's hard to get them right. It'd be pretty neat if I could mark them as "Parameters" or something, and have sliders for their values to adjust and see what happens in realtime, instead of recompiling all the time.

But I'm not sure how it'd look for the code. I mean, it could just be a reader monad, `(Parameters -> Picture)` kinda thing, but that seems like a massive change to the fundementals to have to write everything in monadic style now.

[1]: https://code.world/#PbBOAouMSCYKTj4rseeuyZg

リンクされた Fibonacci tree の例と、現在の CodeWorld の runtime および editor の動作から始めます。ユーザー定義の Parameters をどのように宣言し、スライダーとして公開し、再コンパイルを必要とせずにリアルタイム実行中に適用するかを明らかにします。設計について合意され、調整可能な値を示す動作する例が完成すれば完了です。

索引モデルが issue の本文から書いたものです。