refactor(builtin): dedupe BytesView escaping and template escape writes

- BytesView Show and ToJson now share one streaming `escape_to` helper
  (which prints printable ASCII as-is and renders the rest as `\xHH`),
  and Show renders through a `b"..."` `<+` template
- Char/StringView non-printable `\u{...}` writes collapse into single
  `<+` templates whose hole delegates to a local writer

Inline-writer holes use `\{cb => ...}` so delimiters stay paired and the
template stays readable.

Co-Authored-By: SeekMoon <seekmoon@moonbitlang.com>

Author: bobzhang

builtin/bytesview.mbt

@@ -346,8 +346,10 @@ fn write_byte_hex(logger : &Logger, byte : Byte) -> Unit {
 }
 
 ///|
-pub impl Show for BytesView with fn output(self, logger) {
-  logger.write_string("b\"")
+/// Writes the bytes with printable ASCII kept as-is and everything else
+/// rendered as `\xHH`, without the surrounding `b"`/`"`. Shared by `Show`,
+/// which adds the quotes, and `ToJson`, which does not.
+pub fn BytesView::escape_to(self : BytesView, logger : &Logger) -> Unit {
   for byte in self {
     if byte is (b' '..=b'~') && byte != b'"' && byte != b'\\' {
       logger.write_char(byte.to_char())
@@ -356,7 +358,11 @@ pub impl Show for BytesView with fn output(self, logger) {
       write_byte_hex(logger, byte)
     }
   }
-  logger.write_string("\"")
+}
+
+///|
+pub impl Show for BytesView with fn output(self, logger) {
+  logger <+ "b\"\{cb => self.escape_to(cb)}\""
 }
 
 ///|
@@ -593,14 +599,7 @@ pub fn BytesView::to_owned(self : BytesView) -> Bytes {
 ///|
 pub impl ToJson for BytesView with fn to_json(self) -> Json {
   let sb = StringBuilder()
-  for byte in self {
-    if byte is (b' '..=b'~') && byte != b'"' && byte != b'\\' {
-      sb.write_char(byte.to_char())
-    } else {
-      sb.write_string("\\x")
-      write_byte_hex(sb, byte)
-    }
-  }
+  self.escape_to(sb)
   Json::string(sb.to_string())
 }
 

builtin/char.mbt

@@ -504,9 +504,7 @@ fn Char::escape_to(self : Char, logger : &Logger, quote? : Bool = true) -> Unit
     ' '..='~' => logger.write_char(self)
     _ =>
       if !self.is_printable() {
-        logger.write_string("\\u{")
-        logger.write_string(self.to_hex())
-        logger.write_char('}')
+        logger <+ "\\u{\{cb => cb.write_string(self.to_hex())}}"
       } else {
         logger.write_char(self)
       }

builtin/pkg.generated.mbti

@@ -1170,6 +1170,7 @@ pub fn BytesView::compare(Self, Self) -> Int
 pub fn BytesView::data(Self) -> Bytes
 pub fn BytesView::equal(Self, Self) -> Bool
 pub fn BytesView::equal_to_bytes(Self, Bytes) -> Bool
+pub fn BytesView::escape_to(Self, &Logger) -> Unit
 pub fn BytesView::find(Self, Self) -> Int?
 pub fn BytesView::get(Self, Int) -> Byte?
 pub fn BytesView::get_view(Self, start? : Int, end? : Int) -> Self?

builtin/show.mbt

@@ -191,9 +191,7 @@ fn StringView::escape_to(
       code =>
         if code < ' ' {
           flush_segment(seg, i)
-          logger.write_string("\\u{")
-          logger.write_string(code.to_byte().to_hex())
-          logger.write_char('}')
+          logger <+ "\\u{\{cb => cb.write_string(code.to_byte().to_hex())}}"
           continue i + 1, i + 1
         } else {
           continue i + 1, seg