Increase coverage to 100%

[nventuro]

Coverage has remained at 95% for quite a bit, though I'm not sure if this is really a fault of our test suite, or an integration issue with solidity-coverage (see #1137). We should look into this, figure out what's missing/wrong, and bump the coverage figure.

[frangio]

This has progressed quite a lot. 100% is not doable because of sc-forks/solidity-coverage#269.