lib/strings+test: port join from Hare str... variadic (#19)

selfhost/test/smoke.combined.ww

@@ -1686,6 +1686,47 @@ export fn concat(strs: str...) str = {
 	return r;
 };
 
+// join — fresh allocation with `delim` placed between each element of
+// `strs`. Caller releases with `os.free(r.ptr, r.len: u64)`.
+// ref/hare/strings/concat.ha:46. Hare's `nomem` return is dropped:
+// `os.alloc` aborts on OOM.
+export fn join(delim: str, strs: str...) str = {
+	let total: i32 = 0;
+	let i: i32 = 0;
+	for (i < strs.len) {
+		total += strs[i].len;
+		if (i + 1 < strs.len) { total += delim.len; };
+		i += 1;
+	};
+	let r: str;
+	r.ptr = nil;
+	r.len = 0;
+	if (total == 0) { return r; };
+	let buf: *u8 = os.alloc(total: u64): *u8;
+	let off: i32 = 0;
+	i = 0;
+	for (i < strs.len) {
+		let j: i32 = 0;
+		for (j < strs[i].len) {
+			buf[off + j] = strs[i][j];
+			j += 1;
+		};
+		off += strs[i].len;
+		if (i + 1 < strs.len) {
+			j = 0;
+			for (j < delim.len) {
+				buf[off + j] = delim[j];
+				j += 1;
+			};
+			off += delim.len;
+		};
+		i += 1;
+	};
+	r.ptr = buf;
+	r.len = total;
+	return r;
+};
+
 // sub — borrowed `s[start..end]`. ref/hare/strings/sub.ha:30 is
 // rune-wise; this ww form is byte-wise (no rune iterator yet, planned
 // for commit 2). Clamps out-of-range silently where Hare aborts —