feat: Add SiblingSubgraph::from_node (#1655)

Complementary improvement to #1654.
Creating a k-node subgraph in an n-node graph should ideally be `O(k)`.
However, due to CQCL/portgraph#155 this ends
up being `O(n)`.

For `k=1`, this results in a linear cost overhead.
This PR adds a special case (written by @doug-q) that completely skips
the unnecessary checks.

```
group                         before                                 from_node
-----                         ------                                 ---------
multinode_subgraph/10         1.01     17.7±0.26µs        ? ?/sec    1.00     17.5±0.23µs        ? ?/sec
multinode_subgraph/100        1.00   169.1±11.34µs        ? ?/sec    1.00    168.8±4.37µs        ? ?/sec
multinode_subgraph/1000       1.01      2.3±0.46ms        ? ?/sec    1.00      2.3±0.34ms        ? ?/sec
singleton_subgraph/10         12.26     3.0±0.06µs        ? ?/sec    1.00   245.6±21.24ns        ? ?/sec
singleton_subgraph/100        20.01     4.7±0.06µs        ? ?/sec    1.00    234.4±6.50ns        ? ?/sec
singleton_subgraph/1000       93.34    22.0±0.25µs        ? ?/sec    1.00    235.6±4.93ns        ? ?/sec
```

---------

Co-authored-by: Douglas Wilson <141026920+doug-q@users.noreply.github.com>

hugr-core/src/hugr/views/sibling_subgraph.rs

@@ -211,7 +211,7 @@ impl SiblingSubgraph {
     /// The subgraph signature will be given by the types of the incoming and
     /// outgoing edges ordered by the node order in `nodes` and within each node
     /// by the port order.
-
+    ///
     /// The in- and out-arity of the signature will match the
     /// number of incoming and outgoing edges respectively. In particular, the
     /// assumption is made that no two incoming edges have the same source
@@ -238,6 +238,14 @@ impl SiblingSubgraph {
         checker: &impl ConvexChecker,
     ) -> Result<Self, InvalidSubgraph> {
         let nodes = nodes.into();
+
+        // If there's one or less nodes, we don't need to check convexity.
+        match nodes.as_slice() {
+            [] => return Err(InvalidSubgraph::EmptySubgraph),
+            [node] => return Ok(Self::from_node(*node, hugr)),
+            _ => {}
+        };
+
         let nodes_set = nodes.iter().copied().collect::<HashSet<_>>();
         let incoming_edges = nodes
             .iter()
@@ -265,6 +273,31 @@ impl SiblingSubgraph {
         Self::try_new_with_checker(inputs, outputs, hugr, checker)
     }
 
+    /// Create a subgraph containing a single node.
+    ///
+    /// The subgraph signature will be given by signature of the node.
+    pub fn from_node(node: Node, hugr: &impl HugrView) -> Self {
+        // TODO once https://github.com/CQCL/portgraph/issues/155
+        // is fixed we can just call try_from_nodes here.
+        // Until then, doing this saves a lot of work.
+        let nodes = vec![node];
+        let inputs = hugr
+            .node_inputs(node)
+            .filter(|&p| hugr.is_linked(node, p))
+            .map(|p| vec![(node, p)])
+            .collect_vec();
+        let outputs = hugr
+            .node_outputs(node)
+            .filter_map(|p| hugr.is_linked(node, p).then_some((node, p)))
+            .collect_vec();
+
+        Self {
+            nodes,
+            inputs,
+            outputs,
+        }
+    }
+
     /// An iterator over the nodes in the subgraph.
     pub fn nodes(&self) -> &[Node] {
         &self.nodes