Do not copy typedarrays when converting to Buffer

This PR uses [typedarray-to-buffer](https://github.com/feross/typedarray-to-buffer) to convert the typedarrays into Buffers.

The reason for this is that `new Buffer(typedarray)` results in the typedarray being copied so we should get a nice performance bump by just doing `toBuffer(typedarray)` which only does a reference copy (see feross/buffer#22)