Port clojure.core/reductions

* Fixes squint-cljs#418

resources/squint/core.edn

@@ -142,6 +142,7 @@
   reduce_kv
   reduced
   reduced_QMARK_
+  reductions
   regexp_QMARK_
   remove
   remove_watch

src/squint/core.js

@@ -582,6 +582,32 @@ export function reduce(f, arg1, arg2) {
   return val;
 }
 
+function _reductions(f, arg1, arg2) {
+  const [init, coll] = arg2 === undefined ? [undefined, arg1]: [arg1, arg2];
+  const s = seq(coll);
+  if (init === undefined) {
+    // (reductions f coll)
+    return new LazySeq(function () {
+      return s ? _reductions(f, first(s), rest(s)) : list(f());
+    });
+  } else {
+    // (reductions f val coll)
+    if (reduced_QMARK_(init)) {
+      return list(init.value);
+    }
+    return cons(init, new LazySeq(function () {
+      if (s) {
+        return _reductions(f, f(init, first(s)), rest(s));
+      }
+    }));
+  }
+}
+
+export function reductions(f, arg1, arg2) {
+  f = toFn(f);
+  return _reductions(f, arg1, arg2);
+}
+
 var tolr = false;
 export function warn_on_lazy_reusage_BANG_() {
   tolr = true;