Use Linker in *.wast testing (#1391)

* Use `Linker` in `*.wast` testing

By default `Linker` disallows shadowing previously defined items, but it
looks like the `*.wast` test suites rely on this so this commit adds a
boolean flag to `Linker` as well indicating whether duplicates are
allowed.

* Review comments

* Add a test with a number of recursive instances

* Deny warnings in doctests

* No tabs

crates/api/tests/linker.rs

@@ -64,3 +64,34 @@ fn link_twice_bad() -> Result<()> {
     assert!(linker.define("", "", table.clone()).is_err());
     Ok(())
 }
+
+#[test]
+fn interposition() -> Result<()> {
+    let store = Store::default();
+    let mut linker = Linker::new(&store);
+    linker.allow_shadowing(true);
+    let mut module = Module::new(
+        &store,
+        r#"(module (func (export "export") (result i32) (i32.const 7)))"#,
+    )?;
+    for _ in 0..4 {
+        let instance = linker.instantiate(&module)?;
+        linker.define(
+            "red",
+            "green",
+            instance.get_export("export").unwrap().clone(),
+        )?;
+        module = Module::new(
+            &store,
+            r#"(module
+                (import "red" "green" (func (result i32)))
+                (func (export "export") (result i32) (i32.mul (call 0) (i32.const 2)))
+            )"#,
+        )?;
+    }
+    let instance = linker.instantiate(&module)?;
+    let func = instance.get_export("export").unwrap().func().unwrap();
+    let func = func.get0::<i32>()?;
+    assert_eq!(func()?, 112);
+    Ok(())
+}