Pass positions when evaluating

This includes position information in more places, making debugging
easier.

Before:

```
$ nix-instantiate --show-trace --eval tests/functional/lang/eval-fail-using-set-as-attr-name.nix
error:
       … while evaluating an attribute name

         at «none»:0: (source not available)

       error: value is a set while a string was expected
```

After:

```
error:
       … while evaluating an attribute name

         at /pwd/lang/eval-fail-using-set-as-attr-name.nix:5:10:

            4| in
            5|   attr.${key}
             |          ^
            6|

       error: value is a set while a string was expected
```

src/libexpr/nixexpr.hh

@@ -405,6 +405,7 @@ struct ExprOpNot : Expr
 {
     Expr * e;
     ExprOpNot(Expr * e) : e(e) { };
+    PosIdx getPos() const override { return e->getPos(); }
     COMMON_METHODS
 };