Solidity 0.7.1 support

[vporton]

Testing does not work:

Error: 
    at Deployer._preFlightCheck (/disk2/porton/Projects/bounties/pm-contracts/node_modules/truffle/build/webpack:/packages/deployer/src/deployment.js:179:1)
    at processTicksAndRejections (internal/process/task_queues.js:97:5)
    at /disk2/porton/Projects/bounties/pm-contracts/node_modules/truffle/build/webpack:/packages/deployer/src/deployment.js:282:1
    at Migration._deploy (/disk2/porton/Projects/bounties/pm-contracts/node_modules/truffle/build/webpack:/packages/migrate/Migration.js:68:1)
    at Migration._load (/disk2/porton/Projects/bounties/pm-contracts/node_modules/truffle/build/webpack:/packages/migrate/Migration.js:55:1)
    at Migration.run (/disk2/porton/Projects/bounties/pm-contracts/node_modules/truffle/build/webpack:/packages/migrate/Migration.js:171:1)
    at Object.runMigrations (/disk2/porton/Projects/bounties/pm-contracts/node_modules/truffle/build/webpack:/packages/migrate/index.js:150:1)
    at Object.runFrom (/disk2/porton/Projects/bounties/pm-contracts/node_modules/truffle/build/webpack:/packages/migrate/index.js:110:1)
    at Object.runAll (/disk2/porton/Projects/bounties/pm-contracts/node_modules/truffle/build/webpack:/packages/migrate/index.js:114:1)
    at Object.run (/disk2/porton/Projects/bounties/pm-contracts/node_modules/truffle/build/webpack:/packages/migrate/index.js:79:1)
    at Object.run (/disk2/porton/Projects/bounties/pm-contracts/node_modules/truffle/build/webpack:/packages/core/lib/testing/Test.js:107:1)
Truffle v5.1.56 (core: 5.1.56)
Node v12.16.2
error Command failed with exit code 1.
info Visit https://yarnpkg.com/en/docs/cli/run for documentation about this command.

[cag]

Interesting... I hope this branch is useful for others. I'm not sure whether this would actually get merged and/or published because this version of the code is already kind of cemented on the chain in some sense, but I also guess that this could be useful for people who are trying to talk to the old contracts with the newer Solidity versions. Well, unless their workflow already enables multiple solc versions...

Anyway, conditional tokens are kind of the successor to this set of contracts anyway, so I would consider more the port to 0.7.x there that I think you've posted.

[vporton]

Anyway, conditional tokens are kind of the successor to this set of contracts anyway, so I would consider more the port to 0.7.x there that I think you've posted.

I did port https://github.com/gnosis/conditional-tokens-contracts to 0.7.1 too: gnosis/conditional-tokens-contracts#111 (not enough tested!)