Fix a memory leak with command modules (#2017)

This commit fixes a memory leak that can happen with `Linker::module`
when the provided module is a command. This function creates a closure
but the closure closed over a strong reference to `Store` (and
transitively through any imports provided). Unfortunately a `Store`
keeps everything alive, including `Func`, so this meant that `Store` was
inserted into a cycle which caused the leak.

The cycle here is manually broken by closing over the raw value of each
external value rather than the external value itself (which has a
strong reference to `Store`).

tests/all/linker.rs

@@ -1,4 +1,6 @@
 use anyhow::Result;
+use std::cell::Cell;
+use std::rc::Rc;
 use wasmtime::*;
 
 #[test]
@@ -160,3 +162,64 @@ fn module_interposition() -> Result<()> {
     assert_eq!(func()?, 112);
     Ok(())
 }
+
+#[test]
+fn no_leak() -> Result<()> {
+    struct DropMe(Rc<Cell<bool>>);
+
+    impl Drop for DropMe {
+        fn drop(&mut self) {
+            self.0.set(true);
+        }
+    }
+
+    let flag = Rc::new(Cell::new(false));
+    {
+        let store = Store::default();
+        let mut linker = Linker::new(&store);
+        let drop_me = DropMe(flag.clone());
+        linker.func("", "", move || drop(&drop_me))?;
+        let module = Module::new(
+            store.engine(),
+            r#"
+                (module
+                    (func (export "_start"))
+                )
+            "#,
+        )?;
+        linker.module("a", &module)?;
+    }
+    assert!(flag.get(), "store was leaked");
+    Ok(())
+}
+
+#[test]
+fn no_leak_with_imports() -> Result<()> {
+    struct DropMe(Rc<Cell<bool>>);
+
+    impl Drop for DropMe {
+        fn drop(&mut self) {
+            self.0.set(true);
+        }
+    }
+
+    let flag = Rc::new(Cell::new(false));
+    {
+        let store = Store::default();
+        let mut linker = Linker::new(&store);
+        let drop_me = DropMe(flag.clone());
+        linker.func("", "", move || drop(&drop_me))?;
+        let module = Module::new(
+            store.engine(),
+            r#"
+                (module
+                    (import "" "" (func))
+                    (func (export "_start"))
+                )
+            "#,
+        )?;
+        linker.module("a", &module)?;
+    }
+    assert!(flag.get(), "store was leaked");
+    Ok(())
+}