Make VMDecoder.consume-exactly-bytes use buf8

Instead of blob. This reverts IO::Handle reads to return bufs,
as it did before the handle encoding refactor.

Per: https://irclog.perlgeek.de/perl6-dev/2017-05-26#i_14644075

src/core/Rakudo/Internals/VMBackedDecoder.pm

@@ -43,7 +43,7 @@ my class Rakudo::Internals::VMBackedDecoder is repr('Decoder') {
     }
 
     method consume-exactly-bytes(int $bytes --> Blob) {
-        nqp::ifnull(nqp::decodertakebytes(self, blob8.new, $bytes), Blob)
+        nqp::ifnull(nqp::decodertakebytes(self, buf8.new, $bytes), Blob)
     }
 }