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

Start with the linked Fibonacci tree example and the current CodeWorld runtime and editor behavior. Determine how user-defined Parameters should be declared, exposed as sliders, and applied during realtime execution without requiring recompilation. Done means the design is agreed and a working example demonstrates adjustable values.

Written by the indexing model from the issue text.