Implement a flexible pattern-matching syntax

Just as we currently have `.namedTypes.MemberExpression` and `.builders.memberExpression`, perhaps we could also have an auto-generated `.patterns` namespace containing functions like `.patterns.MemberExpression` for building arbitrary pattern-matching types, e.g.

``` js
var types = require("ast-types");
var p = types.patterns;

var thisFooPattern = p.MemberExpression(
p.ThisExpression(),
p.Identifier("foo")
);

types.traverse(ast, function(node) {
if (thisFooPattern.check(node)) {
console.log("found this.foo:", node);
}
});
```

Start from the ast-types public API exposed through require("ast-types") and examine how namedTypes and builders are generated. Define the scope of the patterns namespace and its check behavior from the MemberExpression example, then add coverage showing that nested patterns match the intended AST nodes.

Written by the indexing model from the issue text.