runtime now looks at the SVf_UTF8 bit on the SV to decide

runtime now looks at the SVf_UTF8 bit on the SV to decide
whether to use widechar semantics; lexer and RE engine continue
to need "use utf8" to enable unicode awareness in literals
and patterns (TODO: this needs to be fixed); $1 et al are marked
SvUTF8 if the pattern was compiled for utf8 (TODO: propagating
it from the data is probably better)

p4raw-id: //depot/perl@4930