Look into leftovers

[xevisalle]

Summary

There are some leftovers in the code that need to be addressed, i.e. FIXMEs, TODOs, etc. In particular:

• https://github.com/dusk-network/plonk/blob/499a70f5da81ade4dbd226f00413a7a17e96f857/src/composer.rs#L507
• This list in WIP