MDEV-6218 Wrong result of CHAR_LENGTH(non-BMP-character) with 3-byte utf8
- Moving get_text() as a method to Lex_input_stream.
- Moving the unescaping part into a separate function,
  this piece of code will later go to /strings most likely.
- Removing Lex_input_string::yytoklen, as it's not needed any more.