Prepare API for parallelization

[cgewecke]

Make it possible to distribute the hit map generated during instrumentation to multiple api.ganache invocations, as necessary.

Make the folder name istanbul writes coverage reports configurable so multiple runs can be combined.

• Refactor api.ganache and test providing client from solcoverjs
• Add api level setters/getters for instrumentationData
• Add istanbulFolder option for custom istanbul destinations
• Add json to the list of default reporters

[codecov-io]

Codecov Report

Merging #436 into buidler-plugin will increase coverage by <.01%.
The diff coverage is 100%.

@@                Coverage Diff                 @@
##           buidler-plugin     #436      +/-   ##
==================================================
+ Coverage           99.21%   99.21%   +<.01%     
==================================================
  Files                  18       18              
  Lines                 760      763       +3     
==================================================
+ Hits                  754      757       +3     
  Misses                  6        6

Impacted Files	Coverage Δ
dist/plugin-assets/plugin.utils.js	97.26% <ø> (ø)	⬆️
lib/ui.js	100% <ø> (ø)	⬆️
dist/plugin-assets/buidler.ui.js	100% <ø> (ø)	⬆️
dist/plugin-assets/buidler.utils.js	100% <ø> (ø)	⬆️
lib/api.js	100% <100%> (ø)	⬆️

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