Live binding incomplete

In adding a unit test for modifying a constant value I noticed a gap.

##### const.js
```js
export const value = 1
```
##### test.js
```js
import { value } from "./const.js"

value = 2 // should throw
```
It should throw since `value` is a `const`, but currently reify doesn't track imported values, so doesn't wrap them in a setter call. *It also defines `value` with either `var` or `let` but that prolly doesn't matter as much because if it were wrapped in a setter call an error would get thrown in the `const` module.*

Start with the import handling exercised by test.js and compare it with the const.js export shown in the report. Add a regression unit test for assigning to the imported value, then run the relevant test suite; done means the assignment throws as described.

Written by the indexing model from the issue text.