Convert to Resources, and other API cleanups.

 - Convert to resources. Use resources instead of `u32`s, remove
   drop functions, `this` arguments, and rename `subscribe-to-*`
   to just `subscribe`, as discussed in WebAssembly/wasi-poll#21.

 - Merge wasi-poll into wasi-io. These two proposals are closely
   related to each other, so it makes sense to have them together.

 - While here, tidy up the poll API, incorporating ideas discussed
   in WebAssembly/wasi-poll#22:

   - Rename `poll-oneoff` to `poll-list`, and add a `poll-one`.

   - Change `poll-oneoff`'s return type from `list<bool>` to `list<u32>`,
     because in the common case, this should allow it to create much
     smaller allocations.

wit/poll.wit

@@ -0,0 +1,34 @@
+package wasi:io
+
+/// A poll API intended to let users wait for I/O events on multiple handles
+/// at once.
+interface poll {
+    /// A "pollable" handle.
+    resource pollable
+
+    /// Poll for completion on a set of pollables.
+    ///
+    /// This function takes a list of pollables, which identify I/O sources of
+    /// interest, and waits until one or more of the events is ready for I/O.
+    ///
+    /// The result `list<u32>` contains one or more indices of handles in the
+    /// argument list that is ready for I/O.
+    ///
+    /// If the list contains more elements than can be indexed with a `u32`
+    /// value, this function traps.
+    ///
+    /// A timeout can be implemented by adding a pollable from the
+    /// wasi-clocks API to the list.
+    ///
+    /// This function does not return a `result`; polling in itself does not
+    /// do any I/O so it doesn't fail. If any of the I/O sources identified by
+    /// the pollables has an error, it is indicated by marking the source as
+    /// being reaedy for I/O.
+    poll-list: func(in: list<pollable>) -> list<u32>
+
+    /// Poll for completion on a single pollable.
+    ///
+    /// This function is similar to `poll-list`, but operates on only a single
+    /// pollable. When it returns, the handle is ready for I/O.
+    poll-one: func(in: pollable)
+}