Refactor hierarchy with goal of Var/Signal interchangeability

- Rename package from `react` to `rescala`
- Replace `Signal` with `DependentSignal`
- New `Signal` trait that combines `Var` and `DependentSignal`
- New `FoldableReactive` trait for functions that only depend on `fold`

- Collect timestamps defs and move into `Reactive`
- Rename `getValue` to `get`, remove `getVal`
- Rename `setValue` to `set`
- Generalize some types, like `Buffer` instead of `ListBuffer`