feat: Provide+implement Replace API

specification/hugr.md

@@ -1236,10 +1236,10 @@ The `Replace` method takes as input:
     $G$ and `SrcNode` in $\Gamma \setminus R$;
   - a list $\mu\_\textrm{out}$ of `NewEdgeSpec` which all have their `SrcNode`in
     $G$ and `TgtNode` in $\Gamma \setminus R$, where `TgtNode` and `TgtPos` describe
-    an existing incoming edge of that kind from a node in $R$.
+    an existing incoming edge of that kind from a node in $S^\*$.
   - a list $\mu\_\textrm{new}$ of `NewEdgeSpec` which all have both `SrcNode` and `TgtNode`
     in $\Gamma \setminus R$, where `TgtNode` and `TgtPos` describe an existing incoming
-    edge of that kind from a node in $R$.
+    edge of that kind from a node in $S^\*$.
 
 Note that considering all three $\mu$ lists together,
    - the `TgtNode` + `TgtPos`s of all `NewEdgeSpec`s with `EdgeKind` == `Value` will be unique

src/builder/build_traits.rs

@@ -73,9 +73,9 @@ pub trait Container {
     fn add_constant(
         &mut self,
         constant: ops::Const,
-        extensions: ExtensionSet,
+        extensions: impl Into<Option<ExtensionSet>>,
     ) -> Result<ConstID, BuildError> {
-        let const_n = self.add_child_node(NodeType::new(constant, extensions))?;
+        let const_n = self.add_child_node(NodeType::new(constant, extensions.into()))?;
 
         Ok(const_n.into())
     }

src/hugr/rewrite.rs

@@ -2,6 +2,7 @@
 
 pub mod insert_identity;
 pub mod outline_cfg;
+pub mod replace;
 pub mod simple_replace;
 
 use crate::{Hugr, HugrView, Node};