Close #16311: Use the _PyUnicodeWriter API in text decoders

 * Remove unicode_widen(): replaced with _PyUnicodeWriter_Prepare()
 * Remove unicode_putchar(): replaced with
   PyUnicodeWriter_Prepare() + PyUnicode_WRITER()
 * When handling an decoding error, only overallocate the buffer by +25%
   instead of +100%