feat: allow overriding of internal functions

[achingbrain]

The interesting bits of this module are the various ways of building DAGs for files and folders (flat, sharded, etc).

Sometimes we want to utilise bits of this logic to build DAGs without having to reimplement big chunks of this module.

This PR allows the user to pass in functions to replace key parts of this import pipeline.

Enables:

• https://github.com/ipfs/js-ipfs-mfs/pull/73
• https://github.com/ipfs/js-ipfs-unixfs-importer/pull/46

[achingbrain]

@mikeal I think this should allow you to accomplish #45 and #46 - see the tests here largely 'inspired' by the tests added to those PRs.

It does it in a way that lets me do ipfs/js-ipfs-mfs#73 using the same mechanism.

[mikeal]

I pulled down the branch and wrote what I need against it and can say for certain that it works and does what I need. +1

[achingbrain]

Released as v0.44.0