string.c: add mrb_utf8_to_buf() to consolidate UTF-8 encoding

Extract duplicated UTF-8 codepoint-to-bytes encoding into a shared
function in src/string.c. Update all gems to use it:

- mruby-sprintf: %c specifier
- mruby-io: putc
- mruby-string-ext: Integer#chr
- mruby-pack: pack("U")
- mruby-compiler: Unicode escapes in parser

Also use existing mrb_utf8len() in io.c for character length detection.

Co-authored-by: Claude <noreply@anthropic.com>

src/string.c

@@ -316,6 +316,39 @@ mrb_gc_free_str(mrb_state *mrb, struct RString *str)
 #define MASK01 0x01010101ul
 #endif
 
+/*
+ * Encode a Unicode codepoint to UTF-8 bytes.
+ * buf must have at least 4 bytes of space.
+ * Returns the number of bytes written (1-4), or 0 for invalid codepoint.
+ */
+mrb_int
+mrb_utf8_to_buf(char *buf, uint32_t cp)
+{
+  if (cp < 0x80) {
+    buf[0] = (char)cp;
+    return 1;
+  }
+  else if (cp < 0x800) {
+    buf[0] = (char)(0xC0 | (cp >> 6));
+    buf[1] = (char)(0x80 | (cp & 0x3F));
+    return 2;
+  }
+  else if (cp < 0x10000) {
+    buf[0] = (char)(0xE0 | (cp >> 12));
+    buf[1] = (char)(0x80 | ((cp >> 6) & 0x3F));
+    buf[2] = (char)(0x80 | (cp & 0x3F));
+    return 3;
+  }
+  else if (cp <= 0x10FFFF) {
+    buf[0] = (char)(0xF0 | (cp >> 18));
+    buf[1] = (char)(0x80 | ((cp >> 12) & 0x3F));
+    buf[2] = (char)(0x80 | ((cp >> 6) & 0x3F));
+    buf[3] = (char)(0x80 | (cp & 0x3F));
+    return 4;
+  }
+  return 0;  /* invalid codepoint */
+}
+
 #ifdef MRB_UTF8_STRING
 
 #define NOASCII(c) ((c) & 0x80)