Encode cid directly in label field rather than try to use structured data

[hannahhoward]

Goals

Because of issues with encoding the CID in a CBOR struct inside a string field for label, but also not wanting to pad things out with JSON, we're switching to encoding the CID directly as Multibase string.

Implementation

• Write label field as a multibase string
  • Use base58 for Cid V0 (only one supported)
  • Use base64 for Cid V1 (base32 is default but we're trying to keep things more compact)

[codecov-commenter]

Codecov Report

Merging #398 into master will decrease coverage by 0.05%.
The diff coverage is 60.00%.

@@            Coverage Diff             @@
##           master     #398      +/-   ##
==========================================
- Coverage   61.97%   61.93%   -0.04%     
==========================================
  Files          43       43              
  Lines        3008     3002       -6     
==========================================
- Hits         1864     1859       -5     
  Misses        987      987              
+ Partials      157      156       -1     

Impacted Files	Coverage Δ
storagemarket/impl/client.go	2.89% <0.00%> (ø)
storagemarket/impl/clientutils/clientutils.go	78.27% <75.00%> (-1.04%)	⬇️

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[ingar]

Looks good