Integrate perlio:

[  9032]
Encode implementation "completion"
Implement and document define_encoding()
Implement and document encoding aliases including define_alias()
Make Encode::XS use define_encoding() rather than back-door.
Move run-time *.enc to separate Encode::Tcl module.
Make 'compile' honour <codeset_name>
Change canonical names of to iso-8859-* and US-ascii.

p4raw-link: @9032 on //depot/perlio: 51ef4e1196e74554150c2d1993b5a0e37f6709c9

p4raw-id: //depot/perl@9034