fix(extgen): correctly handle const blocks to declare iota constants

[alexandre-daubois]

While continuing the work on #2011, I realized that constant declarations have a problem when using iota. I mean, it technically works, but const blocks we not supported which means that setting all constants to iota as shown in the documentation was non-sensical, as iota resets every time outside of const blocks.

So, this is between the bug fix and the feature. To me, it's a bug fix as the behavior wasn't the one intended when creating extgen.