[wip] Begin npyjs.format implementation

[j6k4m8]

Publishing a WIP as I have to step away from this for a little bit and perhaps someone will want to carry this across the finish-line :)

This begins to add a .format function that converts an in-memory typed array back to a .npy-formatted bytestring.

This addresses #15 and borrows some code from @1wheel in #18 (since I can't merge some of the other code in that PR to latest trunk)! Thanks @1wheel :)