reading a trait
```ebnf
trait ::= T_TRAIT T_STRING (T_EXTENDS (NAMESPACE_NAME ',')* NAMESPACE_NAME)? '{' FUNCTION* '}'
```
function() {
    const result = this.node("trait");
    // graceful mode : ignore token & go next
    if (this.token !== this.tok.T_TRAIT) {
      this.error(this.tok.T_TRAIT);
      this.next();
      return null;
    }
    this.next().expect(this.tok.T_STRING);
    let propName = this.node("identifier");
    const name = this.text();
    this.next();
    propName = propName(name);
    this.expect("{");
    const body = this.next().read_class_body();
    return result(propName, body);
  }