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
```

doc/manual/rl-next/source-positions-in-errors.md

@@ -0,0 +1,45 @@
+synopsis: Source locations are printed more consistently in errors
+issues: #561
+prs: #9555
+description: {
+
+Source location information is now included in error messages more
+consistently. Given this code:
+
+```nix
+let
+  attr = {foo = "bar";};
+  key = {};
+in
+  attr.${key}
+```
+
+Previously, Nix would show this unhelpful message when attempting to evaluate
+it:
+
+```
+error:
+       … while evaluating an attribute name
+
+         at «none»:0: (source not available)
+
+       error: value is a set while a string was expected
+```
+
+Now, the error message displays where the problematic value was found:
+
+```
+error:
+       … while evaluating an attribute name
+
+         at bad.nix:4:11:
+
+            3|   key = {};
+            4| in attr.${key}
+             |           ^
+            5|
+
+       error: value is a set while a string was expected
+```
+
+}