wast: first import of a wast format scanner+parser

 - `tokens.go`: Defines the Token type generated by the scanner and several utility functions around it
 - `test.wast`: A .wast test file extracted from [wasm's spec tests](https://github.com/WebAssembly/spec/blob/master/test/core/names.wast)
 - `scanner.go`: Implements the actual scanner and utilty functions regarding runes
 - `scanner_test.go`: A dead simple test case against the `test.wast` file. It doesn't verify correctness yet.
 - Added several new token kind constants to list different typed instructions such as `i64.reinterpret/f64` or `i32.add`
 - Implemented the `(*Scanner).scanTypedReserved()` method to identify well and ill-formed typed instructions
 - Better Scanner test interface, the test suite now takes a `-wast-file` flag that must point to the `.wast` file to Scan and test against.

In order to test:
 - Create (or use a [`spec/test/core`](https://github.com/WebAssembly/spec/tree/master/test/core) ) wast file

Updates #34.

wast/doc.go

@@ -0,0 +1,8 @@
+// Copyright 2017 The go-interpreter Authors.  All rights reserved.
+// Use of this source code is governed by a BSD-style
+// license that can be found in the LICENSE file.
+
+// Package wast provides functions to handle wast files.
+//
+// See https://webassembly.github.io/spec/core/text/
+package wast