Add evalbytes function

Add evalbytes function

This function evaluates its argument as a byte string, regardless of
the internal encoding.  It croaks if the string contains characters
outside the byte range.  Hence evalbytes(" use utf8; '\xc4\x80' ")
will return "\x{100}", even if the original string had the UTF8 flag
on, and evalbytes(" '\xc4\x80' ") will return "\xc4\x80".

This has the side effect of fixing the deparsing of CORE::break under
‘use feature’ when there is an override.