restructure Poll to hold a Table and fd instead of a RefMut<dyn WasiFile>

unfortunately, the borrow checker defeated me: changing the
RwSubscription file form a Ref to a RefMut turned into borrow checker
errors in the impl of the poll_oneoff trait method.

This implementation makes an end run by having Poll hold onto the table
and fd, and borrow the file at the site of use, rather than try to own
the RefMut. I have no idea why this convinces the borrow checker that
anything is different, but it does and I need to get this PR done and
I don't think comprimising on this internal abstraction is worth
fighting against

crates/wasi-common/src/sched.rs

@@ -1,8 +1,8 @@
 use crate::clocks::WasiMonotonicClock;
-use crate::file::WasiFile;
-use crate::Error;
+use crate::table::Table;
+use crate::{Error, ErrorExt};
 use cap_std::time::Instant;
-use std::cell::RefMut;
+use std::collections::HashSet;
 pub mod subscription;
 pub use cap_std::time::Duration;
 
@@ -29,12 +29,18 @@ impl From<Userdata> for u64 {
 }
 
 pub struct Poll<'a> {
+    table: &'a Table,
+    fds: HashSet<u32>,
     subs: Vec<(Subscription<'a>, Userdata)>,
 }
 
 impl<'a> Poll<'a> {
-    pub fn new() -> Self {
-        Self { subs: Vec::new() }
+    pub fn new(table: &'a Table) -> Self {
+        Self {
+            table,
+            fds: HashSet::new(),
+            subs: Vec::new(),
+        }
     }
     pub fn subscribe_monotonic_clock(
         &mut self,
@@ -52,13 +58,31 @@ impl<'a> Poll<'a> {
             ud,
         ));
     }
-    pub fn subscribe_read(&mut self, file: RefMut<'a, dyn WasiFile>, ud: Userdata) {
+    pub fn subscribe_read(&mut self, fd: u32, ud: Userdata) -> Result<(), Error> {
+        if self.fds.contains(&fd) {
+            return Err(
+                Error::invalid_argument().context("Fd can be subscribed to at most once per poll")
+            );
+        } else {
+            self.fds.insert(fd);
+        }
         self.subs
-            .push((Subscription::Read(RwSubscription::new(file)), ud));
+            .push((Subscription::Read(RwSubscription::new(self.table, fd)?), ud));
+        Ok(())
     }
-    pub fn subscribe_write(&mut self, file: RefMut<'a, dyn WasiFile>, ud: Userdata) {
-        self.subs
-            .push((Subscription::Write(RwSubscription::new(file)), ud));
+    pub fn subscribe_write(&mut self, fd: u32, ud: Userdata) -> Result<(), Error> {
+        if self.fds.contains(&fd) {
+            return Err(
+                Error::invalid_argument().context("Fd can be subscribed to at most once per poll")
+            );
+        } else {
+            self.fds.insert(fd);
+        }
+        self.subs.push((
+            Subscription::Write(RwSubscription::new(self.table, fd)?),
+            ud,
+        ));
+        Ok(())
     }
     pub fn results(self) -> Vec<(SubscriptionResult, Userdata)> {
         self.subs