Begin sketching out a new high-level fs API. (#91)

* 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.

src/lib.rs

@@ -28,6 +28,7 @@ mod hostcalls_impl;
 mod sys;
 #[macro_use]
 mod macros;
+pub mod fs;
 mod host;
 pub mod hostcalls;
 mod memory;