proposal: strings: AppendSplitN to reuse destination slice buffer

<pre>I recently found myself forking strings.SplitN to take a destination buffer, to avoid
the slice allocation.

I called it AppendSplitN, with append semantics.  (like the Append funcs in
<a href="http://golang.org/pkg/strconv/)">http://golang.org/pkg/strconv/)</a>

Maybe worth putting in the strings package, although I expect the usual reservations.</pre>
