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/subscription.rs

@@ -1,5 +1,6 @@
 use crate::clocks::WasiMonotonicClock;
-use crate::file::WasiFile;
+use crate::file::{FileCaps, FileEntryMutExt, TableFileExt, WasiFile};
+use crate::table::Table;
 use crate::Error;
 use bitflags::bitflags;
 use cap_std::time::{Duration, Instant};
@@ -12,16 +13,29 @@ bitflags! {
 }
 
 pub struct RwSubscription<'a> {
-    pub file: RefMut<'a, dyn WasiFile>,
+    table: &'a Table,
+    fd: u32,
     status: Cell<Option<Result<(u64, RwEventFlags), Error>>>,
 }
 
 impl<'a> RwSubscription<'a> {
-    pub fn new(file: RefMut<'a, dyn WasiFile>) -> Self {
-        Self {
-            file,
+    /// Create an RwSubscription. This constructor checks to make sure the file we need exists, and
+    /// has the correct rights. But, we can't hold onto the WasiFile RefMut inside this structure
+    /// (Pat can't convince borrow checker, either not clever enough or a rustc bug), so we need to
+    /// re-borrow at use time.
+    pub fn new(table: &'a Table, fd: u32) -> Result<Self, Error> {
+        let _ = table.get_file_mut(fd)?.get_cap(FileCaps::POLL_READWRITE)?;
+        Ok(Self {
+            table,
+            fd,
             status: Cell::new(None),
-        }
+        })
+    }
+    /// This accessor could fail if there is an outstanding borrow of the file.
+    pub fn file(&self) -> Result<RefMut<'a, dyn WasiFile>, Error> {
+        self.table
+            .get_file_mut(self.fd)?
+            .get_cap(FileCaps::POLL_READWRITE)
     }
     pub fn complete(&self, size: u64, flags: RwEventFlags) {
         self.status.set(Some(Ok((size, flags))))