Unset input doesn't work

The REPL is not unsetting input and data rules defined in the current session. This is likely because input and data are transformed into ast.Ref.

```
OPA 0.4.3-dev (commit 7c7dd14, built at 2017-02-10T22:19:57Z)

Run 'help' to see a list of commands.

> input = "x"
> unset input
error: code 0: argument must identify a rule
```