Events sub (#126)

* Make event subscription logic more generic.

* Fix build.

* Add test-node.

* Update deps.

* Address review comments.

test-node/runtime/build.rs

@@ -0,0 +1,26 @@
+// Copyright 2019-2020 Parity Technologies (UK) Ltd.
+// This file is part of substrate-subxt.
+//
+// subxt is free software: you can redistribute it and/or modify
+// it under the terms of the GNU General Public License as published by
+// the Free Software Foundation, either version 3 of the License, or
+// (at your option) any later version.
+//
+// subxt is distributed in the hope that it will be useful,
+// but WITHOUT ANY WARRANTY; without even the implied warranty of
+// MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
+// GNU General Public License for more details.
+//
+// You should have received a copy of the GNU General Public License
+// along with substrate-subxt.  If not, see <http://www.gnu.org/licenses/>.
+
+use wasm_builder_runner::WasmBuilder;
+
+fn main() {
+    WasmBuilder::new()
+        .with_current_project()
+        .with_wasm_builder_from_crates("1.0.11")
+        .export_heap_base()
+        .import_memory()
+        .build()
+}