Merge unicode_eval and evalbytes

This branch adds the unicode_eval and evalbytes feature features.

unicode_eval makes string eval() treats its argument consistently
as characters.

evalbytes enables the evalbytes() function which treats its argument
consistently as bytes.

So the Unicode Bug is hereby fixed for eval.

Source filters actually work logically under both these features.
unicode_eval causes source filters invoked in eval() to croak.
evalbytes actually allows the source code passed to it to be filtered,
instead of leaking source filters out to some other scope like the
historical eval().

'print "Just another Perl hacker,\n"'->CORE'evalbytes