lib/strings: add cut and rcut

selfhost/cmd/w6l/main.combined.ww

@@ -2697,6 +2697,23 @@ export fn remaining_tokens(s: *tokenizer) str = {
 	return frombytes(bytes.remaining_tokens(b));
 };
 
+// cut — split `in` along the first instance of `delim`, returning the
+// portions before and after it. When `delim` is absent the whole input
+// is the first half and the second is empty. Both halves are borrowed
+// from `in`; caller ensures `delim` is non-empty.
+// ref/hare/strings/tokenize.ha:288.
+export fn cut(in: str, delim: str) (str, str) = {
+	let (a, b) = bytes.cut(toutf8(in), toutf8(delim));
+	return (frombytes(a), frombytes(b));
+};
+
+// rcut — like [[cut]] but split along the LAST instance of `delim`.
+// ref/hare/strings/tokenize.ha:302.
+export fn rcut(in: str, delim: str) (str, str) = {
+	let (a, b) = bytes.rcut(toutf8(in), toutf8(delim));
+	return (frombytes(a), frombytes(b));
+};
+
 // rt_ensure is the runtime slice-growth helper invoked by the
 // `append(s, v)` builtin. Direct bind for the same reason as
 // lib/shlex.shlex (appendstr, 16B): the builtin's expansion stores