Eth Collateral Trial 2

[hav-noms]

• Fixes bug https://blog.synthetix.io/bug-disclosure/
• Implemented https://sips.synthetix.io/sccp/sccp-27
• Update Require to check sUSD balance in Depot https://github.com/Synthetixio/synthetix/compare/develop...eth-collat-tests?expand=1#diff-733e929dd68f104f2b18041033bb427eR382

Original SIP https://sips.synthetix.io/sips/sip-35

IOSIRO AUDIT https://gist.github.com/syncikin/2bec003f74c83f44a39c2a26afb2e0ac

[codecov]

Codecov Report

Merging #612 into develop will not change coverage.
The diff coverage is 100.00%.

@@           Coverage Diff            @@
##           develop     #612   +/-   ##
========================================
  Coverage    99.47%   99.47%           
========================================
  Files           45       45           
  Lines         2856     2856           
  Branches       403      403           
========================================
  Hits          2841     2841           
  Misses          15       15           

Impacted Files	Coverage Δ
contracts/EtherCollateral.sol	100.00% <100.00%> (ø)

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