Add structured coroutines and explicit error handling

examples/results.gt

@@ -0,0 +1,18 @@
+package main
+
+import strconv
+import os
+
+fun parse(value: String): Result<Int, Error> {
+    return strconv.atoi(value)
+}
+
+fun changeDirectory(path: String): Result<Unit, Error> {
+    return os.chdir(path)
+}
+
+fun main() {
+    println(parse("42").unwrap())
+    println(parse("invalid").unwrapOr(0))
+    changeDirectory(".").unwrap()
+}