* Begin sketching out a new high-level `fs` API.
This is a very preliminary sketch of #83. It doesn't even compile yet,
but it shows a possible high-level structure of such an API.
* Stub out more functionality.
* Switch from a lazy_static WasiCtx to a borrowed one.
* Reformat some comments.
* Code-quote `Self`.
* Implement error translation for Windows.
* Calls to `fd_close` are now unsafe.
* Implement a few more functions.
