Re-introduce pretty-printing of combining characters.

Re-introduce pretty-printing of combining characters.

This had been removed by commit 07f49ac786e0f1c007eb336e2fb7a572e8405316
("Factorize and optimize `write' for strings and characters.").
Thanks Mike!

* libguile/print.c (write_combining_character): New procedure.
  (write_character): Use it.

* test-suite/tests/chars.test ("basic char handling")["combining accent
  is pretty-printed", "combining X is pretty-printed"]: New tests.

* test-suite/tests/encoding-iso88591.test ("characters")["write A
  followed by combining accent"]: New test.

* test-suite/tests/encoding-utf8.test ("characters")["write A followed
  by combining accent"]: New test.