Generate interface code in new slither.utils.code_generation

[webthethird]

Adds Function.interface_signature_str, Structure.interface_def_str() and Contract.generate_interface().

The new Contract method uses the other two to generate Solidity code for an interface to the contract.

Still isn't quite as clean as the code @glarregay-tob wrote outside of Slither, which I think handles struct return types better.

[webthethird]

pylint is complaining about the following line in Function:

returns = ["address" if isinstance(ret.type, UserDefinedType) and isinstance(ret.type.type, Contract)
           else str(ret.type) for ret in self.returns]

It says E0601: Using variable 'Contract' before assignment (used-before-assignment) but I don't think that's correct.

[0xalpharush]

I think this should not be on the core object and should instead be a utility like generate_interface(Contract)

[webthethird]

I think this should not be on the core object and should instead be a utility like generate_interface(Contract)

That sounds reasonable enough. Do you think I should start a new util in slither.utils? Perhaps one for everything related to code generation?

[0xalpharush]

@webthethird That sounds like a good idea

[webthethird]

Looks like the failing CI checks related to linting are unrelated to this PR:
Pull down action image 'ghcr.io/github/super-linter:slim-v4.9.2'
Error: Docker pull failed with exit code 1

[montyly]

can we add tests? (cc @0xalpharush )

[webthethird]

can we add tests? (cc @0xalpharush )

Would we just test this by having an input .sol file and an expected output .sol file, and doing a diff between the expected and actual output?