Separate Melon contracts from test-related contracts

[Sean Casey]

It's confusing that we have misc third-party and mock contracts that aren't actually used in Melon mixed in with source contracts.

For example, there's no need to have full exchange contracts, the various mock fund components, or things like MaliciousToken.sol that are only used for testing.

We should partition them into separate subfolders within src/ and tests/. For example:

• src/contracts/dependencies/: third party dependencies that we need for Melon to function. E.g., src/contracts/dependencies/exchanges/zeroEx/ with the contracts and libs needed to create the IZeroExV3.sol interface.
• tests/contracts/: misc contracts that we need for our test suite. E.g., tests/contracts/tokens/MaliciousToken.sol

[Travis Jacobs]

As part of 0.5 upgrade #635