Flag functions with empty body as unimplemented

[webthethird]

Flag functions with empty body as unimplemented, unless one or more modifiers are present.

Previously, any function with an empty body, e.g., function hasArgs(uint, uint) public {}, would be marked as implemented, with a CFG that contains a single ENTRY_POINT node and nothing else. This PR leaves the ENTRY_POINT node there, but doesn't set function.is_implemented = True unless the function body contains statements (with the exception of empty functions providing access to a modifier, and special functions fallback/receive/constructor).

Some test files expected these empty functions to be marked implemented, so the test contracts have been tweaked to contain one dummy statement, i.e., uint i = 1;.

[montyly]

What is the reason for this change? I think empty function and unimplemented function are two different concepts

[0xalpharush]

@montyly For #1880, we weren't sure whether function hasArgs(uint, uint) public {} returns(uint x) should generate a RETURN node but it currently has an ENTRY_POINT node. Should we leave is_implemented alone and go ahead and generate a RETURN node?

[montyly]

yes I think generating a return node would be cleaner. However if we do so, we should add the information that the node was not explicit in Solidity, as this information can be useful for further analyses

[webthethird]

yes I think generating a return node would be cleaner.

In that case, we can just close this PR and continue this discussion on #1880