Print result of main (if not Nothing)

[JaroslavTulach]

Pull Request Description

The goal of this request is to simplify hello world and other trivial Enso programs. No need to learn any standard library functions, enough to write:

main = "Hello World!"

and the result is going to be printed:

enso$ ./built-distribution/enso-engine-0.0.0-dev-linux-amd64/enso-0.0.0-dev/bin/enso --run hello.enso 
'Hello World!'

the result is only printed, if it is not Nothing. E.g. if the last statement is IO.print ... (which returns Nothing), then no value is printed at the end by the launcher.

Why is this Important?

The simplification of hello world is a nice side-effect, but it is not the primary reason why this functionality is important.

There is a task to Add missing @TruffleBoundary to make Enso code compilation more effective. The only way to verify that there are no missing @TruffleBoundary annotations is to build the engine-runner project with GraalVM Native Image - #3638. However just building the engine isn't enough, we also need to test it by executing a sample script and verify it computes the right output. However, at current stage, the Native Image compiled Enso executable cannot load standard library - e.g. it cannot use Standard.Base.IO.println to print anything...

With here in proposed functionality we can invoke the script, compute its result and let the engine-runner infrastructure print the output. We can compile all of that into native executable right now - e.g. we will be able to properly test the Add missing @TruffleBoundary task in our CI.

Checklist

Please include the following checklist in your PR:

• All code conforms to the
  Scala,
  Java,
  and
  Rust
  style guides.
• All code has been tested:
  • If GUI codebase was changed: Enso GUI was tested when built using BOTH
    ./run ide build and ./run ide watch.

[4e6]

Looks good. Only request is to change the output to stdout

[hubertp]

Same request as the others