Make `dec_slice_of` return a slice rather than a pointer-length pair,
freeing its users from having to call the unsafe `slice::from_raw_parts`.
This requires splitting `dec_slice_of` and `dec_ptr` into mut and
non-mut versions, and reorganizing poll_oneoff a little to avoid
borrow-checker errors -- decoded slices do alias the main memory, so
make sure functions only need one or the other.
The upstream wasi/core.h header switched from anonymous unions to unions
named `u`. This patch updates wasm32.rs to reflect this, as was done in
wasmtime 5b77f952.
