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
```

tests/functional/lang/eval-fail-attr-name-type.err.exp

@@ -0,0 +1,20 @@
+error:
+       … while evaluating the attribute 'puppy."${key}"'
+
+         at /pwd/lang/eval-fail-attr-name-type.nix:3:5:
+
+            2|   attrs = {
+            3|     puppy.doggy = {};
+             |     ^
+            4|   };
+
+       … while evaluating an attribute name
+
+         at /pwd/lang/eval-fail-attr-name-type.nix:7:17:
+
+            6| in
+            7|   attrs.puppy.${key}
+             |                 ^
+            8|
+
+       error: value is an integer while a string was expected

tests/functional/lang/eval-fail-attr-name-type.nix

@@ -0,0 +1,7 @@
+let
+  attrs = {
+    puppy.doggy = {};
+  };
+  key = 1;
+in
+  attrs.puppy.${key}

tests/functional/lang/eval-fail-call-primop.err.exp

@@ -0,0 +1,12 @@
+error:
+       … while calling the 'length' builtin
+
+         at /pwd/lang/eval-fail-call-primop.nix:1:1:
+
+            1| builtins.length 1
+             | ^
+            2|
+
+       … while evaluating the first argument passed to builtins.length
+
+       error: value is an integer while a list was expected

tests/functional/lang/eval-fail-call-primop.nix

@@ -0,0 +1 @@
+builtins.length 1

tests/functional/lang/eval-fail-not-throws.err.exp

@@ -0,0 +1,18 @@
+error:
+       … in the argument of the not operator
+
+         at /pwd/lang/eval-fail-not-throws.nix:1:4:
+
+            1| ! (throw "uh oh!")
+             |    ^
+            2|
+
+       … while calling the 'throw' builtin
+
+         at /pwd/lang/eval-fail-not-throws.nix:1:4:
+
+            1| ! (throw "uh oh!")
+             |    ^
+            2|
+
+       error: uh oh!

tests/functional/lang/eval-fail-not-throws.nix

@@ -0,0 +1 @@
+! (throw "uh oh!")

tests/functional/lang/eval-fail-using-set-as-attr-name.err.exp

@@ -0,0 +1,11 @@
+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

tests/functional/lang/eval-fail-using-set-as-attr-name.nix

@@ -0,0 +1,5 @@
+let
+  attr = {foo = "bar";};
+  key = {};
+in
+  attr.${key}