Show function names in error messages

Functions in Nix are anonymous, but if they're assigned to a
variable/attribute, we can use the variable/attribute name in error
messages, e.g.

while evaluating `concatMapStrings' at `/nix/var/nix/profiles/per-user/root/channels/nixos/nixpkgs/pkgs/lib/strings.nix:18:25':
...

1 files changed, 4 insertions, 0 deletions

diff --git a/src/libexpr/nixexpr.hh b/src/libexpr/nixexpr.hh
index 86a7bfd506a0..8256605b5cac 100644
--- a/src/libexpr/nixexpr.hh
+++ b/src/libexpr/nixexpr.hh
@@ -63,6 +63,7 @@ struct Expr
     virtual void bindVars(const StaticEnv & env);
     virtual void eval(EvalState & state, Env & env, Value & v);
     virtual Value * maybeThunk(EvalState & state, Env & env);
+    virtual void setName(Symbol & name);
 };
 
 std::ostream & operator << (std::ostream & str, Expr & e);
@@ -197,6 +198,7 @@ struct Formals
 struct ExprLambda : Expr
 {
     Pos pos;
+    Symbol name;
     Symbol arg;
     bool matchAttrs;
     Formals * formals;
@@ -208,6 +210,8 @@ struct ExprLambda : Expr
             throw ParseError(format("duplicate formal function argument `%1%' at %2%")
                 % arg % pos);
     };
+    void setName(Symbol & name);
+    string showNamePos();
     COMMON_METHODS
 };