go/common/prettyprint: Rename FractionBase10() to QuantityFrac()

[tjanez]

I think it is a better name for this function as it more clearly captures the specific relation to Quantity type.

Follow up from: #3129 (comment).

[codecov]

Codecov Report

Merging #3131 into master will decrease coverage by 0.08%.
The diff coverage is 100.00%.

@@            Coverage Diff             @@
##           master    #3131      +/-   ##
==========================================
- Coverage   69.00%   68.92%   -0.09%     
==========================================
  Files         374      374              
  Lines       37000    37000              
==========================================
- Hits        25532    25502      -30     
- Misses       8232     8251      +19     
- Partials     3236     3247      +11     

Impacted Files	Coverage Δ
go/common/prettyprint/quantity.go	100.00% <100.00%> (ø)
go/staking/api/prettyprint.go	100.00% <100.00%> (ø)
go/worker/compute/executor/committee/fault.go	68.96% <0.00%> (-8.05%)	⬇️
go/storage/api/root_cache.go	72.41% <0.00%> (-6.90%)	⬇️
go/worker/storage/service_external.go	47.31% <0.00%> (-6.46%)	⬇️
go/runtime/tagindexer/tagindexer.go	68.47% <0.00%> (-4.35%)	⬇️
go/runtime/host/sandbox/sandbox.go	67.28% <0.00%> (-4.09%)	⬇️
go/consensus/tendermint/services.go	78.90% <0.00%> (-3.91%)	⬇️
go/storage/mkvs/tree.go	89.28% <0.00%> (-3.58%)	⬇️
go/runtime/host/protocol/connection.go	62.16% <0.00%> (-3.48%)	⬇️
... and 31 more

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[Yawning]

Ok sure.