fix: copy bytes from shared memory for fd_write

Previous Wiggle changes (bytecodealliance#5229, bytecodealliance#5264) made it possible to access slices
of WebAssembly linear memory from Wiggle but limited the interface so
that slices of shared memory could only be accessed either `unsafe`-ly
or via copying (`GuestPtr::to_vec`). This change modifies `fd_write` to
unconditionally copy the bytes to be written before passing them to
Rust's standard library in an `IoSlice`. This is likely not the optimal
solution but enables further `wasi-threads` development. In the future
this commit should probably change to conditionally copy the bytes when
shared memory is detected and expand to fix up all the `expect` errors
of the same kind in `preview_1.rs`.

crates/wasi-common/src/snapshots/preview_1.rs

@@ -351,16 +351,12 @@ impl wasi_snapshot_preview1::WasiSnapshotPreview1 for WasiCtx {
         let f = self.table().get_file(u32::from(fd))?;
         let f = f.get_cap(FileCaps::WRITE)?;
 
-        let guest_slices: Vec<wiggle::GuestSlice<u8>> = ciovs
+        let guest_slices: Vec<Vec<u8>> = ciovs
             .iter()
             .map(|iov_ptr| {
                 let iov_ptr = iov_ptr?;
                 let iov: types::Ciovec = iov_ptr.read()?;
-                Ok(iov
-                    .buf
-                    .as_array(iov.buf_len)
-                    .as_slice()?
-                    .expect("cannot use with shared memories; see https://github.com/bytecodealliance/wasmtime/issues/5235 (TODO)"))
+                Ok(iov.buf.as_array(iov.buf_len).to_vec()?)
             })
             .collect::<Result<_, Error>>()?;