Add check command

[ollef]

Add a sixten check command to the command-line interface that runs the typechecker but doesn't go further, which will be faster than using compile when you just want to see if something typechecks.

[brandonhamilton]

I'm interested in building this and have a question on what the preferable implementation of this feature should be:

• Should it return after running the only the frontend on each module, which would require additional options passed through and changes to be made to the Processor.Files module.

• Or, run the full frontend and backend processing and then return before writing the modules out to disk and running llvm. This would need only a minor change to the processor, but would be doing more work.

I would assume the first option, but just wanted to check before submitting the implementation.

[ollef]

Yeah, the first option is what I had in mind.

I'll have a look at your PR now.