Initial AST validation for using-for

[ferranbt]

This PR adds the initial AST validation for the using statement. This PR only has tests for the 3 parsing scenarios while the Solidity repo has dozens of them. I was wondering how do you want to approach that on solar. Should I create sub-directories like they do? Merge them on the same file? Should I copy/paste the whole syntax e2e tests from Solidity? Is it enough with basic tests and we do not need as many as Solidity has?

[DaniPopes]

As long as there is at least one test per error message it's fine; if you want to add more syntax tests in the same file, otherwise the other ones in that directory also include type checking which we'll add as we implement them

[ferranbt]

otherwise the other ones in that directory also include type checking which we'll add as we implement them

Would you include those in the same file as well?

[DaniPopes]

sure yeah, but you might need to split it into multiple files if a parsing error occurs because it won't continue checking the next lines

[ferranbt]

sure yeah, but you might need to split it into multiple files if a parsing error occurs because it won't continue checking the next lines

Ok, will keep in mind for next time. This PR is ready for review again.

[DaniPopes]

LGTM, needs rebase