Add encoding conversion (transcoding) from UTF-8 to CESU-8
and back. CESU-8 is an encoding similar to UTF-8, but encodes
codepoints above U+FFFF as two surrogates, these surrogates
again being encoded as if they were UTF-8 codepoints. This
preserves the same binary sorting order as in UTF-16. It is
also somewhat similar (although not exactly identical) to an
encoding used internally by Java.

This completes issue #15995.

enc/trans/cesu_8.trans: Add encoding conversion from/to CESU-8
test/ruby/test_transcode.rb: Add tests for above