bug#33057: Guile's reader assumes the port encoding is ASCII-compatible

[Mark H Weaver]

In several places, Guile's reader assumes that the port encoding is
ASCII-compatible.  For example:

* 'scm_token' reads raw bytes and passes them to the CHAR_IS_DELIMITER
  macro to check for delimiters.

* 'scm_read_mixed_case_symbol' checks for the (optional) postfix keyword
  syntax by comparing the final _byte_ read by 'read_token' with ':'.

* 'scm_read_character' uses 'read_token' to read raw bytes up to the
  next delimiter, and if it returns a single byte with value 0-127, it
  is interpreted as the associated ASCII character regardless of the
  encoding.

* 'scm_read_semicolon_comment' reads bytes until it finds one equal to
  the ASCII code for '\n'.

There may be other problems as well.

       Mark