Move unixfs chunking logic to ipfs-car, carbites, and w3up-client

[olizilla]

The code for creating a CAR for an upload should be in a re-usable library that folks can build on in there own apps.

The existing web3.storage client library uses ipfs-car and carbites to handle car creation and car splitting, so any improvements that are needed in those libs to make it possible for w3up-client to defer most of the work to them should be prioritised.

TL:DR

• at least move all the CAR creation logic to w3up-client where it can be imported and built on in users apps. No one wants to depend on a cli module from an app.
• better move the generic parts to ipfs-car for car creation and carbites for car splitting, then consume those functions from w3up-client.

[JeffLowe]

Possible duplicate of web3-storage/w3up-client#27 I left a note there to confirm it is a duplicate, and if yes then close that one out in favor of this one.