subbuf-rw only works on Buf:D
subbuf-rw only works on Buf:D

See http://irclog.perlgeek.de/perl6/2016-02-26#i_12106997

- Since we're now using a Buf only, we can use nqp::splice
- This means no longer making copies in memory
- so this should be significantly faster for bigger Bufs

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