More lib files, more functional

Still missing that 'attack' fn, though :(

lib/combinators.ts

@@ -0,0 +1,7 @@
+export const tap = (fn) => (input) => {
+  fn(input);
+  return input;
+};
+
+export const doIf = (fn, pred) => (input) =>
+  pred ? fn(input) : input;