In operator on dict in implement statement produces error

```
entity Test:
    dict attributes
end

implementation test for Test:

end

implement Test using test when "foo" in self.attributes

Test(attributes={"foo": 42})
```

Result
```
  File "/home/bart/workspace/inmanta/src/inmanta/ast/constraint/expression.py", line 452, in _bin_op
    raise Exception("Operand two of 'in' can only be a list (%s)" % arg2[0])
KeyError: 0
```