Add structured coroutines and explicit error handling

internal/lang/data_class_test.go

@@ -53,7 +53,7 @@ import json encoding.json
 data class Request(var email: String, private val traceId: String)
 
 fun email(body: ByteSlice): String {
-    val request = json.decode<Request>(body)
+    val request = json.decode<Request>(body).unwrap()
     return request.email
 }
 `)