replace go-merkledag with go-fetcher

[acruikshank]

Motivation

As part of a broader effort to use IPLD prime in IPFS, we want to start replacing read only usages of go-merkledag with go-fetcher. This PR replaces the usage of merkledag.Walk with fetcher.Session.BlockAll to retrieve all links from a potentially remote Dag.

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[willscott]

looks good. we'll need to wait for the pending ipld-prime PR to land since it changes a bit the encoder interface, right?

[willscott]

we may consider releasing this as a v1.0 (semver major breaking release) and maintaining a branch for any needed patch fixes of the v0.x implementation.

[Stebalien]

Note: I've reverted this PR in #31 to avoid breaking changes on master till they're ready. Please re-file it against a branch if you need it for something.