Integrate perlio:

[ 16246]
Have :encoding() default to perlqq style fallbacks.
Add test for that.

[ 16245]
Use (file PerlIO::Layer 'perlio') rather than $Config{useperlio}
as gate on layer tests which fail on -Dusesfio
(ext/XS/Typemap/Typemap.t still fails on -Dusesfio)

[ 16244]
Builds under -Uuseperlio
Uses PUSHSTACK/POPSTACK in :encoding and :Via
p4raw-link: @16246 on //depot/perlio: 1982da4048668033f4bb857b02c690606711056a
p4raw-link: @16245 on //depot/perlio: 6b5da1a3dd326d0fe0f59ec1ea7b9b5d72b2a49e
p4raw-link: @16244 on //depot/perlio: 24f59afc531955e5c3768c6023fb2ac247d6939d

p4raw-id: //depot/perl@16247