feat(wasi)!: make most WasiFile methods take &mut self (#3901)

1. This makes it easier for implementors to deal with internal APIs.
2. This matches the signatures of the WASI Snapshot traits.

Although it is likely true that these methods would have to become
immutable in order to implement threading efficiently, threading will
impact a large number of existing traits. So this change is practical
for now with an already-unavoidable change required for threading.

Signed-off-by: Nathaniel McCallum <nathaniel@profian.com>

crates/wasi-common/tokio/tests/poll_oneoff.rs

@@ -20,7 +20,7 @@ async fn empty_file_readable() -> Result<(), Error> {
     let d = workspace.open_dir("d").context("open dir")?;
     let d = Dir::from_cap_std(d);
 
-    let f = d
+    let mut f = d
         .open_file(false, "f", OFlags::CREATE, false, true, FdFlags::empty())
         .await
         .context("create writable file f")?;