add fennelrepl package

Runs fennel using a Lua compiled with the same options as the
host system, and with packages set up so it can find all the local
Lua packages

To shorten the dev feedback loop further, allows FENNEL_PATH to be set
on the command line so you can point directly it at the Fennel sources
for some library you're working against instead of having to run
nix-build and compile them to Lua

default.nix

@@ -58,6 +58,7 @@ in {
       borderVm.build.vm
       go-l2tp
       min-copy-closure
+      fennelrepl
     ];
   };
 }

pkgs/default.nix

@@ -57,4 +57,5 @@
   kernel-modules  = callPackage ./kernel-modules {};
   odhcp-script = callPackage ./odhcp-script {};
   fennel = callPackage ./fennel {};
+  fennelrepl = callPackage ./fennelrepl {};
 }

pkgs/fennelrepl/default.nix

@@ -0,0 +1,34 @@
+{
+  runCommand
+, runtimeShell
+, fetchurl
+, lib
+, luaPackages
+, lua
+, writeScriptBin
+, linotify
+, anoia
+, fennel
+}:
+let packages = [
+      linotify
+      anoia
+      fennel
+    ];
+    join = ps: builtins.concatStringsSep ";" ps;
+    luapath = join (builtins.map (f: "${f}/share/lua/${lua.luaversion}/?.lua") packages);
+    luacpath = join (builtins.map (f: "${f}/lib/lua/${lua.luaversion}/?.so") packages);
+
+in writeScriptBin "fennelrepl" ''
+    #!${lua}/bin/lua
+    package.path = ${lib.strings.escapeShellArg luapath} .. ";" .. package.path
+    package.cpath = ${lib.strings.escapeShellArg luacpath} .. ";" .. (package.cpath or "")
+    local fennel = require "fennel"
+    fennel.install()
+    local more_fennel = os.getenv("FENNEL_PATH")
+    if more_fennel then
+        fennel.path = more_fennel .. ";" .. fennel.path
+    end
+    print("path", fennel.path)
+    fennel.repl()
+  ''