DO NOT MERGE Incremental inliner

**Do not merge this PR yet**, it is obviously in a very rough state.

But now the algorithm should be complete enough that things won't change much anymore, except cleanups. So I'm posting it for the eager interested reader.
It must be here, because a lonely `VarDef` can not even be in a block, e.g.,
```scala
if (test) {
  val x = 5
}
```
will result in a `VarDef` directly in the `If`, without `Block` in between.