Use arena allocation instead of reference counting for `Module`s to fix memory leaks from `Rc` cycles.
A module references its module children and its import resolutions, and an import resolution references the module defining the imported name, so there is a cycle whenever a module imports something from an ancestor module.
For example,
```rust
mod foo { // `foo` references `bar`.
fn baz() {}
mod bar { // `bar` references the import.
use foo::baz; // The import references `foo`.
}
}
```
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