Improve farming code readability [SC-929]

[byshape]

• Renamed FarmAccounting => Farming, UserAccounting => Rewards
• Renamed FarmingLib.startFarming => updateFarmData, stopFarming => cancelFarming
• Renamed FarmAccounting.startFarming => update, stopFarming => cancel
• Moved the functionality related to the distributor to a separate abstract contract Distributor
• Added documentation to 3 main contracts: FarmingPlugin, FarmingPool, MultiFarmingPlugin
• Updated the code to be more in line with the Solidity Style Guide
• Updated README.md

[codecov]

Codecov Report

Attention: 4 lines in your changes are missing coverage. Please review.

Files	Coverage Δ
contracts/Distributor.sol	100.00% <100.00%> (ø)
contracts/FarmingLib.sol	100.00% <100.00%> (ø)
contracts/FarmingPlugin.sol	100.00% <100.00%> (ø)
contracts/FarmingPool.sol	100.00% <100.00%> (ø)
contracts/accounting/Farming.sol	100.00% <100.00%> (ø)
contracts/accounting/Rewards.sol	100.00% <100.00%> (ø)
contracts/MultiFarmingPlugin.sol	69.84% <78.94%> (-4.49%)	⬇️

📢 Thoughts on this report? Let us know!.

[ZumZoom]

Closing in favour of #72, #73 and #74