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`).
cb7b1aadcc