Add expression functions with context boundaries

examples/http_context.gt

@@ -0,0 +1,12 @@
+package main
+
+import fmt
+import http net.http
+
+fun handle(request: *http.Request) = withContext(request.context()) {
+    fmt.println(coroutineContext())
+}
+
+fun main() {
+    println("HTTP handlers can establish their request context with withContext")
+}