ds-help: add helper functions to convert from Cid to DsKey and the reverse

[kevina]

In the filestore (#2634) i needed to convert from Cid to DsKey and the reverse in several places so I created two helper functions to that effect.

[whyrusleeping]

Maybe these should be in the dshelper package?

[kevina]

I take it you mean thirdparty/ds-help. I thought about that, but it will add a dependency to that small package and also these methods are used in the same places the blockstore is used so I thought the blockstore package was a better place to avoid having yet another import. I can move it though, just give the word.

[whyrusleeping]

@kevina the thing is that your helper methods would be useful in nearly every place that that package is used.

[kevina]

No problem, I'll move it.

[kevina]

@whyrusleeping, moved

[whyrusleeping]

LGTM

[kevina]

@whyrusleeping tests passed. Marking a RFM.