Add structured coroutines and explicit error handling

examples/showcase.gt

@@ -13,25 +13,6 @@ class PrefixGreeter(val prefix: String): Greeter {
     }
 }
 
-worker Counter {
-    var count = 0
-
-    fun increment() {
-        count += 1
-    }
-
-    fun value(): Int {
-        return count
-    }
-}
-
-fun risky(input: String): String {
-    if (input == "boom") {
-        throw "boom requested"
-    }
-    return input
-}
-
 fun main() {
     val greeter: Greeter = PrefixGreeter("hello")
     println(greeter.greet("gotlin"))
@@ -44,35 +25,25 @@ fun main() {
     val upper = strings.ToUpper("gotlin")
     fmt.Println("interop:", upper)
 
-    val result = runCatching({ risky("boom") })
-    if (result.isSuccess()) {
-        println("runCatching: success")
-    } else {
-        println("runCatching:")
-        println(result.exceptionOrNull())
-    }
-
-    val maybe: any = null
+    val maybe: String? = null
     if (maybe == null) {
         println("null check works")
     }
 
-    val counter = Counter()
-    counter.increment()
-    counter.increment()
-    fmt.Println("worker value:", counter.value())
-
-    val ready = Channel<String>()
-    go {
-        select {
-            after(120) -> ready.send("timer fired")
+    runBlocking {
+        val ready = Channel<String>()
+        launch {
+            delay(120)
+            ready.send("timer fired")
+        }
+        select {
+            ready -> println("channel says: " + it)
         }
-    }
-    select {
-        ready -> println("channel says: " + it)
-    }
 
-    select {
-        every(50) -> println("one periodic tick")
+        val answer = async<Int> {
+            delay(50)
+            return 42
+        }
+        fmt.Println("async value:", answer.await())
     }
 }