buf.slice() should do parent tracking to match node api behavior

[feross]

See for description of the problem: toots/buffer-browserify#19

From the node docs:

buf.slice([start], [end])#
start Number, Optional, Default: 0
end Number, Optional, Default: buffer.length
Returns a new buffer which references the same memory as the old, but offset and cropped by the start (defaults to 0) and end (defaults to buffer.length) indexes. Negative indexes start from the end of the buffer.

Modifying the new buffer slice will modify memory in the original buffer!