Merge js-ipfs-{core pieces} into js-ipfs repo

[daviddias]

Following #2222, the proposal is to:

Merge into js-ipfs, the following modules:

• js-ipfs-mfs
• js-ipfs-repo
• js-ipfs-block-service
• js-ipfs-unixfs-exporter
• js-ipfs-unixfs-importer

[hugomrdias]

@achingbrain do we have another issue to track this ?

[achingbrain]

I'm not sure that we want to merge any more modules into the main repo at this time.

• ipfs-mfs is in already
• I don't think ipfs-repo is really high-churn enough, though we might fold that and the migration tool into one
• The block service should just go away as it's only purpose is to delegate to either repo.blockstore or bitswap, which delegates to repo.blockstore so let's just configure components with one or the other during init/start (post or as part of refactor: remove proxy api object and detect initialisation state #2762)
• The importer/exporter are similarly low churn, kind of happy with them where they are right now

We can, of course revisit these should the above things change.

[hugomrdias]

cool lets close this then.