Kdl compilation genereates needless/unused terms

[developedby]

The kind to kdl compiler currently generates code that sometimes contains lets that are never used, with an unbound variable, lets binding a variable to another (useless indirection) and dups where at least one of the bound variables is unused.

The first two I'm certain it happens and will add some examples later, the third one I'm not so sure, but vaguely remember it happening before.

[algebraic-dev]

I think that it could be an optimization pass for all backends not only for KDL.