multiple Packr instances and useBuffer()

[endel]

As the encoding Buffer is tied to the module scope (target), when using multiple Packr instances along with .useBuffer() - that Packr instance is not ensured to be using the correct Buffer.

A workaround would be to always call .useBuffer() prior to encoding, but that implies in creating a new DataView instance before each encode.

[kriszyp]

Last commit will cache the DataView (as is done elsewhere in the code), so you can performantly switch buffers back and forth with useBuffer() (without new DataView each time).