Feature: UFCS on blocks

[dvdvgt]

This PR adds UFCS for blocks such that you can write { x => x + 42 }.foo { y => x * 2 }, for example.

[dvdvgt]

Current caveat: When expecting a statement, a UFCS call with blocks is not allowed. For example

val x = { f1 }.foo { f2 }
              ^
expected ; but found .

fails since it expects a BlockStmt. This only works when explicitly returning:

val x = return { f1 }.foo { f2 }

[jiribenes]

Related discussion: #845

[jiribenes]

What about, say,

interface State[T] {
  def get(): T
  def set(x: T): Unit
}

def increment() { st: State[T] }: Unit = {
  val current = st.get()
  st.set(current + 1)
}

def st: State[T] = ...

increment() {st}

Can we now write st.increment() or does it have to be {st}.increment()? Or is it not supported at all?