String.join can be replaced with a faster version

src/transforms/replaceStringFunctions.ts

@@ -0,0 +1,40 @@
+import ts, { isIdentifier } from 'typescript';
+import { ast } from './utils/create';
+
+const $elm$core$String$join = `
+var $elm$core$String$join = F2(function (sep, strs) {
+  if (!strs.b) {
+    return "";
+  }
+  var acc = strs.a;
+  strs = strs.b;
+
+  for (; strs.b; strs = strs.b) {
+    acc = acc + sep + strs.a;
+  }
+
+  return acc;
+};
+`;
+
+
+const replacements = {
+  $elm$core$String$join,
+};
+
+export const replaceStringFunctions: ts.TransformerFactory<ts.SourceFile> = (
+  context
+) => (sourceFile) => {
+  const visitor = (node: ts.Node): ts.VisitResult<ts.Node> => {
+    if (ts.isVariableStatement(node)) {
+      const name = node.declarationList.declarations[0]?.name;
+      if (isIdentifier(name) && name.text in replacements) {
+        const key = name.text as keyof typeof replacements;
+        return ast(replacements[key]);
+      }
+    }
+    return ts.visitEachChild(node, visitor, context);
+  };
+
+  return ts.visitNode(sourceFile, visitor);
+};