Implement int extension

[sapir]

I tried to fix the FIXME in zext. Would this be acceptable?

Also, how should I add tests?

[antoyo]

For the test, you can either add it to this file or you can create another file in that directory.

[antoyo]

Did you want to add a test or would you want to merge this as is?

[sapir]

I did try to add a test and found that the test passes either way, so maybe I should change the PR to just add the test instead of changing the code?

[antoyo]

You mean, to make sure the test fails without the implementation of int extension?
If so, yes, you can push a test, remove the implementation to make sure the test fails and push back the implementation.