Initial version of nix-repl

This program interactively reads a Nix expression from the user,
evaluates it, and prints the result.

default.nix

@@ -0,0 +1,10 @@
+with import <nixpkgs> { };
+
+runCommand "nix-repl"
+  { buildInputs = [ readline nixUnstable boehmgc ]; }
+  ''
+    mkdir -p $out/bin
+    g++ -O3 -Wall -o $out/bin/nix-repl ${./nix-repl.cc} \
+      -I${nixUnstable}/include/nix -L${nixUnstable}/lib/nix \
+      -lexpr -lmain -lreadline
+  ''