Change the len parameter of fadvise to filesize. (#7)

* Change the `len` parameter of `fadvise` to `filesize`.

Fadvise can apply to regions of files larger than the linear memory
address space, so it wants a `filesize` (which is u64) rather than a
`size` (which is u32 on wasm32).

* Update the host implementation accordingly.

host/src/filesystem.rs

@@ -8,7 +8,7 @@ impl wasi_filesystem::WasiFilesystem for WasiCtx {
         &mut self,
         fd: wasi_filesystem::Descriptor,
         offset: wasi_filesystem::Filesize,
-        len: wasi_filesystem::Size,
+        len: wasi_filesystem::Filesize,
         advice: wasi_filesystem::Advice,
     ) -> HostResult<(), wasi_filesystem::Errno> {
         todo!()