Factorize and optimize `write' for strings and characters.

Factorize and optimize `write' for strings and characters.

According to `write.bm', this makes `write' 2.6 times faster for strings.

* libguile/print.c (iprin1): Use `write_character' when
  `SCM_WRITINGP (pstate)' and `SCM_CHARP (exp)' or `scm_is_string (exp)'.
  (scm_i_charprint): Remove.
  (display_character, write_character): New functions.
  (scm_write_char): Use `display_character' instead of
  `scm_i_charprint'.

* libguile/print.h (scm_i_charprint): Remove declaration.

* benchmark-suite/benchmarks/write.bm: New file.

* benchmark-suite/Makefile.am (SCM_BENCHMARKS): Add
  `benchmarks/write.bm'.