Implement sprintf("%c") for UTF-8.

* sprintf("%c") is changed to accept a string for which String#size returns
  1, even if it is longer than one byte, and to convert a Fixnum via
  Fixnum#chr (possibly returning more than one byte).  Thus, if the UTF-8
  gem is in use, a character will be understood as a single UTF-8 character.

* The change to sprintf depends on the implementation of Fixnum#chr added
  to mrbgems/mruby-string-utf8/src/string.c.

This should work with any other gem that implements a multibyte encoding, as
long as it implements String#size and Fixnum#chr as appropriate.
Update the Fixnum#chr test for UTF-8.
Test for UTF-8 as mattn suggests.

The string is "Hello world" in Japanese.  (hat tip:  Google Translate.)
Merge pull request #1756 from chasonr/sprintf-utf8

Implement sprintf("%c") for UTF-8.