POC: use shared ArrayBuffer for `nextv()`

[vweevers]

This makes db.iterator() with buffer encoding as fast as an iterator with utf8 encoding. The approach is:

1. In each nextv() call, create a std::vector<char> to hold the raw data of multiple entries
2. Copy LevelDB slices directly into that vector with memcpy()
3. Create an ArrayBuffer backed by the vector
4. In JS, split it into Buffers, each backed by the same ArrayBuffer but using a different offset.

Apart from this being an incomplete implementation (it makes utf8 slower because the C++ side is buffer-only meaning JS has to transcode buffer to utf8), the approach has a downside: if userland code keeps a reference to just one of the Buffers, the entire ArrayBuffer is kept alive too. I.e. it costs memory.

For now this PR is just a reference. The ideal solution (for this particular bottleneck) sits somewhere in between. For example, I might take just the ArrayBuffer concept, to replace use of napi_create_buffer_copy().