Require main to always return unit

[b-studios]

In the LLVM backend, we experienced problems with boxing of the result of main. Here, we require that main always returns Unit and check this.

Related to #541

[b-studios]

This breaks effekt.WebTests since they rely on the (only) exported function main to return a result that can be inspected.

We could potentially use a mutable reference in JS to sidechannel the result and compare it this way for testing purposes.

[jiribenes]

This is truly also relevant to exit in #541. Currently it's defined there for jsNode, but I don't know what it should do on jsWeb. Perhaps we could use it for the side-channeling?

[b-studios]

There are some more tests that fail now. However, the most interesting one is the stdlib/test/test which uses the unit testing library that returns a boolean.

[jiribenes]

However, the most interesting [failing test] is the stdlib/test/test which uses the unit testing library that returns a boolean.

I saw this coming :^), #541 introduces def mainSuite(name: String) { body: => Unit / Test }: Unit which uses exit explicitly instead of returning a boolean which is supposed to be used exactly like def main() = mainSuite("my tests") { <tests> } :)

The problem is that it's not exactly great for how we use stdlib tests right now in the compiler — exit(1) would mean that the Scala test fails completely.