disallow ix access into objects

Don't allow numeric ix access into objects
```
Asteroid Version 1.1.4
(c) University of Rhode Island
Type "asteroid -h" for help
Press CTRL-D to exit
ast> structure A with data a end
ast> let a = A(1).
ast> let b = a@1.  <<< this should throw an error
error: 1 is not a member
ast>
```
