Move contracts out of the root directory

[come-maiz]

We are currently using the contracts root directory as a bin where we put everything that doesn't fit anywhere else. This is wrong, if we can't find a good place for a contract, it's likely that it shouldn't be in our repo.

We should move all the contracts that are directly in the contracts directory.

[come-maiz]

This is what we have discussed so far:

• AddressUtils.sol -> utils/Address.sol
• ECRecovery.sol -> cryptography/ECDSA.sol
• LimitBalance.sol -> remove it.
• MerkleProof.sol -> utils/MerkleProof.sol
• ReentrancyGuard.sol -> utils/ReentrancyGuard.sol
• Bounty.sol -> maybe move it to bounties/BreakInvariantBounty.sol and promote the idea to get new bounties, or move it to examples, or remove it.

Note that it's also a problem to just make utils the new garbage/all purpose bin. It might be a good step, but we should avoid filling it with many things. And if a better directory comes to mind, we should use it instead of utils.