Export the exports of the last actor of a program, if present

[nomeata]

This finally gives us a way to compile to a WebAssembly module that
exports more than just a start function.

This patch also add the ability to invoke methods from the test suite,
simply by adding lines like

//CALL hello

as seen in hello-world-message.as

Two regressions:

• Previously we required programs to have type (), i.e. end with a
  unit expression. This is no longer true, and I simply removed the
  check (in Pipeline). One could check for “unit or actor” at some
  point.
• Previously, the test suite would add a

  print("Top-level code done.\n")
  

  to all test cases, to better recognize when the top-level did not go
  through. This doesn't work any more, so I just removed it.