Avoid memory explosion when stress testing bindgen

[kennykerr]

The windows-bindgen crate's bindgen method is meant to be called once per build but during stress testing, it can be called numerous times from the same process leading to an explosion of memory usage. The winmd reader does an awkward dance with the winmd files to avoid a complex lifetime model. I hope to improve this over time but at least with this change the internal lifetime of the reader and files are scoped to the bindgen function call and thus greatly reduces memory usage during build validation.

[riverar]

Thanks!