For controlling what interfaces an optimization must preserve (typically functions, but also perhaps the entrypoint if a DFG)

We may want ability to tell passes not to waste compute time on optimizing other parts, but this is less of an issue - few classical compilers bother very much! Instead the preferred way is to split your Hugr into several, optimize them separately/differently, and then link them back together.

Hence, propose to combine these two concepts into a single enum:

#[non_exhaustive]
enum OptimizationScope<N> {
   SpecificFuncs (Vec<N>), // heavily discouraged because serialization renumbers - maybe remove altogether and require setting visibility??
    // maybe SpecificPublicFunctions(Vec<String>), etc. etc.

   AllPublic (include_entrypoint: bool), // true has no effect if entrypoint==module-root

   All (include_entrypoint: bool), // true has no effect if entrypoint==module-root

   Aggressive, // if entrypoint is module root, then AllPublic(false); else just entrypoint

   Local, // use entrypoint, do not recurse (possibly error on module-entrypoint?)
}

Default == All(true) as this is pretty conservative. Indeed, perhaps too conservative: All means DeadFunctionRemoval can't remove unused private functions! (It makes no sense to allow removal of functions but not alteration; a pass could create a new function, change all calls to use that, and remove the old one!)

Add ComposablePass::set_scope (breaking change as new pass method)

Note SpecificFuncs requires them all to be top-level/module children, so cannot express arbitrary entrypoint

There is some method to_nodes<H: HugrView>(&OptimizationScope, &H) -> Vec<H::Node> (all elements of which are not guaranteed to be module children), but this might be hidden. (Or, this might want to return 2*Vec: places to examine/transform, and funcs whose semantics to preserve - the former must be a superset of the latter but usually plus reachable funcs - e.g. if we are preserving interface of public funcs, we might still butcher up the private ones according to how they are used.)