refactor: Extract utility functions into utils library

These are reusable functions, and I want to avoid duplicate implementations
of them.

nix/excludeOrgSubtreesOnHeadlines.nix

@@ -2,27 +2,11 @@
 pred:
 with builtins;
 let
-  dropWhile = import ./dropWhile.nix;
-
-  splitListWith = import ./splitWith.nix;
-
-  isHeadline = s: substring 0 1 s == "*";
-
-  genericHeadlineRegexp = "(\\*+)[[:space:]].+";
-
-  getHeadlineLevel = headline:
-    stringLength (head (match genericHeadlineRegexp headline));
-
-  prependOptionalStars = n: rest:
-    if n == 0
-    then rest
-    else prependOptionalStars (n - 1) ("\\*?" + rest);
-
-  makeSubtreeEndRegexp = outlineLevel:
-    prependOptionalStars (outlineLevel - 1) "\\*[[:space:]].+";
-
-  dropTillSubtreeEnd = level:
-    dropWhile (s: !(isHeadline s && match (makeSubtreeEndRegexp level) s != null));
+  inherit (import ./utils.nix)
+    dropTillSubtreeEnd
+    getHeadlineLevel
+    splitListWith
+    isHeadline;
 
   go_ = cut:
     cut.before

nix/utils.nix

@@ -0,0 +1,25 @@
+with builtins;
+let
+  genericHeadlineRegexp = "(\\*+)[[:space:]].+";
+
+  prependOptionalStars = n: rest:
+    if n == 0
+    then rest
+    else prependOptionalStars (n - 1) ("\\*?" + rest);
+in  
+rec {
+  dropWhile = import ./dropWhile.nix;
+
+  splitListWith = import ./splitWith.nix;
+
+  isHeadline = s: substring 0 1 s == "*";
+
+  getHeadlineLevel = headline:
+    stringLength (head (match genericHeadlineRegexp headline));
+
+  makeSubtreeEndRegexp = outlineLevel:
+    prependOptionalStars (outlineLevel - 1) "\\*[[:space:]].+";
+
+  dropTillSubtreeEnd = level:
+    dropWhile (s: !(isHeadline s && match (makeSubtreeEndRegexp level) s != null));
+}