DCA improvements

I think there is room for improvement on our DCA. For example, the screenshot below shows how rustc considers: A, T and f to be dead, because no "live code" actually uses them.

We, on the other hand, consider the mere presence of impls enough for them to not be dead.

Remove the impl and we now correctly mark this as dead. But the error message is not as clear as rustc's, which says the struct is never "constructed".

This is all done at sway-core/src/control_flow_analysis/dead_code_analysis.rs and seems a candidate for "good first issue".