Add structured coroutines and explicit error handling

examples/nullability.gt

@@ -0,0 +1,18 @@
+package main
+
+data class User(var email: String)
+
+fun emailOrMissing(user: User?): String {
+    if (user == null) { return "missing" }
+    return user.email
+}
+
+fun optionalEmail(user: User?): String? {
+    return user?.email
+}
+
+fun main() {
+    val user: User? = User("user@example.com")
+    println(emailOrMissing(user))
+    println(user!!.email)
+}