forceValue: make pos mandatory

- Make passing the position to `forceValue` mandatory,
  this way we remember people that the position is
  important for better error messages
- Add pos to all `forceValue` calls

src/nix/eval.cc

@@ -81,7 +81,7 @@ struct CmdEval : MixJSON, InstallableCommand
 
             recurse = [&](Value & v, const Pos & pos, const Path & path)
             {
-                state->forceValue(v);
+                state->forceValue(v, pos);
                 if (v.type() == nString)
                     // FIXME: disallow strings with contexts?
                     writeFile(path, v.string.s);