UTF-X encoding invariance for Encode:

UTF-X encoding invariance for Encode:
 - move Encode::utf8_encode to utf8::encode (likewise decode,upgrade,downgrade,valid)
 - move the XS code for those to universal.c (so in miniperl)
 - add utf8::unicode_to_native and its inverse to allow EBCDIC to work in true unicode.
 - change ext/Encode/compile to use above.
 - Fix t/lib/encode.t for above
 - Teach t/lib/b.t to expect -uutf8
 - In utf8.c look for SWASHNEW rather than just utf8:: package to see if
   utf8.pm is needed.

p4raw-id: //depot/perlio@9198