Reading trait alias
```ebnf
trait_use_alias ::= namespace_name ( T_DOUBLE_COLON T_STRING )? (T_INSTEADOF namespace_name) | (T_AS member_flags? T_STRING)
```
name list : https://github.com/php/php-src/blob/master/Zend/zend_language_parser.y#L303
trait adaptation : https://github.com/php/php-src/blob/master/Zend/zend_language_parser.y#L742
function() {
    const node = this.node();
    let trait = null;
    let method;

    if (this.is("IDENTIFIER")) {
      method = this.node("identifier");
      const methodName = this.text();
      this.next();
      method = method(methodName);
    } else {
      method = this.read_namespace_name();

      if (this.token === this.tok.T_DOUBLE_COLON) {
        this.next();
        if (
          this.token === this.tok.T_STRING ||
          (this.php7 && this.is("IDENTIFIER"))
        ) {
          trait = method;
          method = this.node("identifier");
          const methodName = this.text();
          this.next();
          method = method(methodName);
        } else {
          this.expect(this.tok.T_STRING);
        }
      } else {
        // convert identifier as string
        method = method.name;
      }
    }

    // handle trait precedence
    if (this.token === this.tok.T_INSTEADOF) {
      return node(
        "traitprecedence",
        trait,
        method,
        this.next().read_name_list()
      );
    } else if (this.token === this.tok.T_AS) {
      // handle trait alias
      let flags = null;
      let alias = null;
      if (this.next().is("T_MEMBER_FLAGS")) {
        flags = this.read_member_flags();
      }

      if (
        this.token === this.tok.T_STRING ||
        (this.php7 && this.is("IDENTIFIER"))
      ) {
        alias = this.node("identifier");
        const name = this.text();
        this.next();
        alias = alias(name);
      } else if (flags === false) {
        // no visibility flags and no name => too bad
        this.expect(this.tok.T_STRING);
      }

      return node("traitalias", trait, method, alias, flags);
    }

    // handle errors
    this.expect([this.tok.T_AS, this.tok.T_INSTEADOF]);
    return node("traitalias", trait, method, null, null);
  }