Make interruptWhen a no-op with the Sync compiler

[rossabaker]

Addresses #2371. Solution first discussed in #2650.

Before, compiling a concurrent stream with the Sync compiler would fail quickly and pessimistically. Now we proceed with no-op interruption. We can now successfully process more streams with this compiler at the expense of non-termination of some.

This is helpful when constraining to Sync when Async would have brought a more powerful compiler, or translating a Concurrent stream to a Synchronous effect like Doobie's ConnectionIO.

[rossabaker]

I'm trying to understand the subtle ways Sync and Concurrent compilers might diverge. I'm surprised this test fails: I thought if interruptWhen isn't processed, the left would not be interrupted and still emit the 1. Instead, we get Nil. I'm not disappointed that it behaves more like Concurrent, but I'm confused.

    Deferred[IO, Unit].flatMap { rightHalt =>
      val left = Stream.eval(rightHalt.get).drain ++ Stream.sleep[IO](100.millis) ++ Stream.emit(1)
      val right = Stream.eval(rightHalt.complete(())).drain
      compileWithSync(left.mergeHaltBoth(right)).toList
    }.assertEquals(List(1))

[mpilquist]

@rossabaker That one works b/c merge internally compiles and runs both streams. Hence, the compilation of the internal left/right is using the Concurrent instance picked up from merge.