[L-06] Add slither check for potential function clashes in Configurat…

…or contract (compound-finance#430)

Add slither analysis check

Comet balance constraint + Constraint and Transfer scenarios + Comet …

…Solidity bug fix (compound-finance#247)

This PR includes the following changes:

-Constraint for setting the Comet balance of an actor
-More transfer scenarios and new set of scenarios for testing constraints
-Fixing a Solidity underflow bug that was caught by the transfer scenarios
-Librarifying some code between the BalanceConstraint and -CometBalanceConstraint

Example of CometBalanceConstraint usage:

```
scenario(
  'Comet#comet balance constraint',
  {
    cometBalances: {
      albert: { $base: 100, $asset0: 10 }, // in units of asset, not wei
    },
  },
```