print UniChar and UTF8[S] as UTF-8 sequences

[stepancheg]

print* functions with UniChar or UTF8[S] parameters print UTF-8 string. Before these commits print* functions printed repr of values.

Pull request contains 4 commits:

• convert utf8 test to test framework
• make utf-8 encoder visible outside of utf8 package
• overload printTo(stream, UniChar)
• overload printTo(stream, UTF8[S])