Upgrade to Solidity 0.6

[nicholaspai]

Changes to NPM dependencies:

• Truffle bumped to 5.1.14 from 5.1.4 (>= 5.1.5 supports solc 0.6)
• OpenZeppelin bumped to v3 which supports solc 0.6
• Prettier Plugin Solidity bumped to alpha.42 from alpha.36 (>= alpha.40 seemingly supports some of solc 0.6)
• Solidity coverage bumped to 0.7.2 which addresses some solidity 0.6 changes
• Patched @drizzle/store fork to account for 0.6 ABI changes and bumped version

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Had to prettier-ignore several solidity code blocks because the solidity plugin that we use for prettier does not support some of the breaking 0.6 changes. See #969 :

• Inline assembly call in Governor that attempts to call built-in opcode gas() (previously gas) Fixed by this PR in alpha.42
• All override methods that implement virtual methods

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Changes to contract implementations

• No longer can adjust an array's size by dynamically settings its length, for example:

 uint newLength = partyMap[parties[i]].derivatives.push(derivativeAddress);
 partyMap[parties[i]].derivativeIndex[derivativeAddress] = newLength - 1;

becomes

partyMap[parties[i]].derivatives.push(derivativeAddress);
uint newLength = partyMap[parties[i]].derivatives.length;
partyMap[parties[i]].derivativeIndex[derivativeAddress] = newLength - 1;

• Array.push() no longer returns a value

---

TODO:

• Solidity Docgen needs to be upgraded to work with solc 0.6
• Temporarily removed the following from CI:
  • Slither does not work with 0.6 Add slither back to CI once it supports solc 0.6 #976
• Drizzle store seemingly incompatible with 0.6 ABI's, I raised an issue

Fixes #815

[nicholaspai]

@mrice32 @ptare @chrismaree can you take a look at the PR notes and let me know what you think before I proceed further?

[mrice32]

@ptare could you take a look at this?

[nicholaspai]

I added empty tests for docs, echidna and slither in the CI's config.yaml

[nicholaspai]

Can't merge this yet, seems like some dapps are breaking

[mrice32]

Looks great -- two nits.

[mrice32]

LGTM!

[chrismaree]