strings: preserve UTF-8 and boundary invariants

lib/strings/replace_test.ww

@@ -7,6 +7,7 @@ package strings_test;
 
 import strings;
 import os;
+import test;
 
 // ref/hare/strings/replace.ha:46 (#4). Hare rows 1-5 (target found
 // once / multiple times / shorter / longer / multibyte) plus ww rows
@@ -108,3 +109,7 @@ import os;
 	};
 };
 
+@test fn replace_empty_needle_aborts() void = {
+	test.expectabort();
+	strings.replace("abc", "", "x");
+};