constant folding: FuncDefns and FuncDecls are not considered needed

[doug-q]

In find_needed_nodes

On reflection this is not entirely true. We consider all incoming edges of a needed node to come from needed nodes. So a FuncDefn/Decl is needed if it's called by a needed node.

The issue is that "main" (or more generally, public functions) are not considered needed.

Constant folding already special cases the inputs of "main", we should at least special case that "main" is needed.

[acl-cqc]

Is it worth generalizing this to "establish a convention/API for indicating the entry-points to a Hugr" i.e. not just for constant folding but generally? (DCE/DeadFuncElim, etc. etc.) - see also #1752

[acl-cqc]

and/or does it suffice to change this line:

hugr/hugr-passes/src/const_fold.rs

Line 152 in cad7484

q.push_back(h.root());

to look for main if the root is not a DFG/function but a module?