Fixes: #787

Summary

This PR adds a general migration contract that takes address of the Safe, SafeL2 and fallback handler contracts during deployment. The contract allows Safe to update the Singleton at address(0).

• Uses error strings rather than error types because Solidity version 0.7.6 doesn't support it.

As of now tests cover below migration paths:

• 1.3.0 to 1.5.0
• 1.3.0 to 1.4.1
• 14.1 to 1.5.0

See SafeMigration.spec.ts to see how tests are organised. Do share if you any thoughts to better run same tests on different migration paths.

The migration contract stores address of the Safe singletons and fallback handler rather than using code hash and requiring the user to provide singleton address as described in the issue. The reason being as follows:

Checking codehash of the target singleton means user has to provide the address of the target singleton. Also, checking code hash has higher gas costs.

The only argument for using code hash for upgrades is that it also allows unofficial singletons to be used for migration using official migration contract. But, users/projects can also deploy their own version of migration contract by providing singleton addresses in the constructor and have similar security guarantees as the official migration contract.

Changes in PR

• Create a general migration contract which is not tightly bound to any specific Safe version
• Update tests
• Remove other migration contract as this PR supersedes it

Unlike Safe150Migration.sol, this new contract does not check if slot(0) of the contract stores an address having some non-empty code. I think this check is not need because this contract is not intended to be used in general by other proxy contracts and checking slot(0) value is only a partially correct way. Would like to know thought of others.