chore: change from callbacks to promises

[reconbot]

Works ipfs/js-ipfs#1670

[vmx]

Thanks @reconbot for your contribution! I sadly can't merge this right away, as the IPLD project is taking the change of also making other API changes to the format. ipld/interface-ipld-format#50 contains more information about those.

My plan is to get the top-level API working first (ipld/js-ipld#185) and then updates all those formats. That will be a lot of work and any help will be appreciated once we get there.

I'll do a proper review of this PR, once I know what changes will exactly be needed (I'll port ipld-dag-pb first.

Sorry for not communicating this more clearly earlier.

[reconbot]

Yeah no worries, looks like I should close this in the meantime. =)

[codecov]

Codecov Report

❗ No coverage uploaded for pull request base (master@c370777). Click here to learn what that means.
The diff coverage is 84.44%.

@@            Coverage Diff            @@
##             master      #38   +/-   ##
=========================================
  Coverage          ?   93.68%           
=========================================
  Files             ?        7           
  Lines             ?      285           
  Branches          ?        0           
=========================================
  Hits              ?      267           
  Misses            ?       18           
  Partials          ?        0

Impacted Files	Coverage Δ
src/util/commit.js	95.89% <62.5%> (ø)
src/util/tree.js	93.93% <71.42%> (ø)
src/util.js	89.74% <78.94%> (ø)
src/util/tag.js	95.23% <80%> (ø)
src/resolver.js	93.1% <92.15%> (ø)

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[reconbot]

Just reread your comment, I'll keep this open for you =)

[vmx]

This is superseded by #51. Though thanks for your contribution @reconbot!