feat!: Return a node mapping in HugrInternals::region_portgraph

[aborgna-q]

Closes #2158

Annoyingly, we still need to return a FlatRegion (or some other concrete wrapper) here so that references to the graph implements some of petgraph's traits (like IntoNodeIdentifiers) which are normally implemented for &'_ G. There's no way to write that in a GAT constraint :/
We'll have to wait for the big petgraph rewrite to clean that.

BREAKING CHANGE: HugrInternals::region_portgraph now also returns a node mapping between hugr and portgraph nodes

[hugrbot]

This PR contains breaking changes to the public Rust API.

cargo-semver-checks summary

--- failure trait_associated_type_added: non-sealed public trait added associated type without default value ---

Description:
A non-sealed trait has gained an associated type without a default value, which breaks downstream implementations of the trait
      ref: https://doc.rust-lang.org/cargo/reference/semver.html#trait-new-item-no-default
     impl: https://github.com/obi1kenobi/cargo-semver-checks/tree/v0.41.0/src/lints/trait_associated_type_added.ron

Failed in:
trait associated type hugr_core::hugr::internal::HugrInternals::RegionPortgraph in file /home/runner/work/hugr/hugr/PR_BRANCH/hugr-core/src/hugr/internal.rs:28
trait associated type hugr_core::hugr::internal::HugrInternals::RegionPortgraphNodes in file /home/runner/work/hugr/hugr/PR_BRANCH/hugr-core/src/hugr/internal.rs:37

[codecov]

Codecov Report

Attention: Patch coverage is 95.45455% with 1 line in your changes missing coverage. Please review.

Project coverage is 82.10%. Comparing base (8e36cdc) to head (3370bf9).
Report is 1 commits behind head on main.

Files with missing lines	Patch %	Lines
hugr-core/src/hugr/views/impls.rs	0.00%	1 Missing ⚠️

Additional details and impacted files

@@           Coverage Diff           @@
##             main    #2164   +/-   ##
=======================================
  Coverage   82.10%   82.10%           
=======================================
  Files         227      227           
  Lines       39795    39805   +10     
  Branches    35894    35904   +10     
=======================================
+ Hits        32672    32682   +10     
  Misses       5301     5301           
  Partials     1822     1822           

Flag	Coverage Δ
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rust	81.72% <95.45%> (+<0.01%)	⬆️

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[lmondada]

Looks amazing, thanks @aborgna-q!

I've tried to remove the methods

    #[inline]
    fn to_portgraph_node(&self, node: impl NodeHandle<Self::Node>) -> portgraph::NodeIndex {
        node.node().into_portgraph()
    }
    #[inline]
    fn from_portgraph_node(&self, index: portgraph::NodeIndex) -> Self::Node {
        index.into()
    }

in HugrInternals and all their uses, but I've hit a roadblock in the implementation of HugrMut::insert_from_view. This turns the view into a portgraph view and then inserts that.

Do you have a plan on how to rewrite that? It's not obvious to me how an implementation of insert_from_view would look if we remove the fn portgraph method. In any case, it seems big enough to be its own PR.

[aborgna-q]

There's a TODO in insert_hugr_internal:

TODO: We may need to replicate portgraph::LinkMut::insert_graph at the
hugr level to avoid calling hierarchy() here.

hugr/hugr-core/src/hugr/hugrmut.rs

Lines 569 to 571 in 851e396

	// TODO: We may need to replicate `portgraph::LinkMut::insert_graph` at the
	// hugr level to avoid calling `hierarchy()` here.
	let other_portgraph =

We'll need to implement that to remove those uses :/