libexpr: add set-to-string error frame

this is much more useful than a frame pointing to the set but claiming
it as a function being called. if the function is actually at fault we
will now point to its attribute, although the position may be slightly
wrong if __toString was defined from set updates or builtin functions.

Change-Id: Ib3eb237a276d94426d9c6fc0e26eea72382d34a2

doc/manual/rl-next/tostring-frames.md

@@ -0,0 +1,60 @@
+---
+synopsis: 'Implicit `__toString` now have stack trace entries'
+issues: []
+cls: [3055]
+category: Improvements
+credits: [horrors]
+---
+
+Coercion of attribute sets to strings via their `__toString` attribute now produce stack
+frames pointing to the coercion site and the attribute definition. This makes locating a
+coercion function error easier as the fault location is now more likely to be presented.
+
+Previously:
+```
+nix-repl> builtins.substring 1 1 "${{ __toString = self: throw ''bar''; }}"
+error:
+       … while calling the 'substring' builtin
+         at «string»:1:1:
+            1| builtins.substring 1 1 "${{ __toString = self: throw ''bar''; }}"
+             | ^
+
+       … caused by explicit throw
+         at «string»:1:48:
+            1| builtins.substring 1 1 "${{ __toString = self: throw ''bar''; }}"
+             |                                                ^
+
+       error: bar
+```
+
+Now:
+```
+nix-repl> builtins.substring 1 1 "${{ __toString = self: throw ''bar''; }}"
+error:
+       … while calling the 'substring' builtin
+         at «string»:1:1:
+            1| builtins.substring 1 1 "${{ __toString = self: throw ''bar''; }}"
+             | ^
+
+       … while converting a set to string
+         at «string»:1:25:
+            1| builtins.substring 1 1 "${{ __toString = self: throw ''bar''; }}"
+             |                         ^
+
+       … from call site
+         at «string»:1:29:
+            1| builtins.substring 1 1 "${{ __toString = self: throw ''bar''; }}"
+             |                             ^
+
+       … while calling '__toString'
+         at «string»:1:42:
+            1| builtins.substring 1 1 "${{ __toString = self: throw ''bar''; }}"
+             |                                          ^
+
+       … caused by explicit throw
+         at «string»:1:48:
+            1| builtins.substring 1 1 "${{ __toString = self: throw ''bar''; }}"
+             |                                                ^
+
+       error: bar
+```