Add a FileTree builder

Builder that takes a mapping `path -> target` (as a Nickel record) and
builds a tree of symlinks corresponding to that mapping (the
documentation of the function itself might be a bit clearer).

lib/builders.ncl

@@ -179,4 +179,51 @@ in
         env.content = content,
       }
         | NixpkgsPkg,
+  FileTree
+    | doc m%"
+    Turn a record of derivations the form
+
+    ```
+    {
+      "path1" = drv1,
+      "path2/with/subpath" = drv2,
+    }
+    ```
+
+    to a single derivation whose content is a tree of symlinks of the form
+
+    ```
+    |- path1 -> drv1
+    |- path2
+        |- with
+            |- subpath -> drv2
+    ```
+  "%
+    =
+      {
+        input_paths | { _ : NixString },
+        # TODO: Sanity-check the inputs
+        # - Escape the values
+        # - Raise a proper error is there's some overlap between the
+        #     paths
+        env.buildCommand = nix-s%"
+          mkdir -p "$out"
+          cd "$out"
+          %{
+            std.record.to_array input_paths
+            |> std.array.fold_right
+            (fun { field, value } acc =>
+            nix-s%"
+              mkdir -p "$(dirname '%{field}')"
+              ln -s '%{value}' '%{field}'
+
+              %{acc}
+            "%
+            )
+            ""
+          }
+      "%,
+        version = "0.0",
+      }
+        | NixpkgsPkg
 }

tests/FileTree.ncl

@@ -0,0 +1,30 @@
+let organist = import "../lib/nix.ncl" in
+let paths = {
+  "bin/hello" = organist.lib.import_file "tests/hello.sh",
+  "README.md" =
+    organist.builders.TextFile
+    & {
+      name = "README",
+      content = "Hello package",
+    },
+}
+in
+let file_tree =
+  organist.builders.FileTree
+  & {
+    name = "foobar",
+    input_paths = paths,
+  }
+in
+organist.builders.NixpkgsPkg
+& {
+  name = "test-filetree",
+  version = "0.1",
+  env.buildCommand =
+    nix-s%"
+    [[ $(realpath '%{file_tree}/bin/hello') == $(realpath '%{paths."bin/hello"}') ]]
+    [[ $(realpath '%{file_tree}/README.md') == $(realpath '%{paths."README.md"}') ]]
+    echo "OK" > $out
+  "%
+      | organist.contracts.NixString,
+}

tests/TextFile.ncl

@@ -1,6 +1,7 @@
 let organist = import "../lib/nix.ncl" in
-let
-  someText = organist.builders.TextFile & {
+let someText =
+  organist.builders.TextFile
+  & {
     name = "foo",
     content = "Heyio"
   }
@@ -16,4 +17,3 @@ organist.builders.NixpkgsPkg
   "%
       | organist.contracts.NixString,
 }
-

tests/main.ncl

@@ -1,4 +1,5 @@
 {
   ShellApplication = import "ShellApplication.ncl",
-  TextFile = import "TextFile.ncl"
-}
+  TextFile = import "TextFile.ncl",
+  FileTree = import "FileTree.ncl",
+}