Defining scales with function

Add an API for defining scale as a actual function.

Something like that:

```
scale = continuous {
x -> Color.rgb(0, 0, x / 100 * 256)
}

scale = categorical { x ->
when (x) {
"" -> LineType.SOLID
}
}
```

Implementation = direct applying + identity scale (#402), but there's problems with column id, should we save both original and transformed column, etc.

Review the direct-applying and identity-scale work in issue #402 first. Define how continuous and categorical function scales should be exposed, including how column IDs and original versus transformed columns are handled; done means the API and its data-flow semantics are agreed and implemented.

Written by the indexing model from the issue text.