Add pseudo-streams. (bytecodealliance#29)

* Add pseudo-streams.

This add a pseudo-stream type to the wasi-poll interface, and adds ways
to obtain streams from command invocation and from files. In the future,
it can support sockets too. With this, `command` takes streams for
stdin/stdout, rather than filesystem descriptors.

Streams support reading and writing, as well as skipping,
repeated-element writing, and splicing from one stream to another. And
there are `subscribe-*` functions to produce pseudo-futures from
pseudo-streams, allowing them to be polled.

This makes the polyfill somewhat more complex, but this is largely due
to the polyfill being tied to the preview1 API.

This replaces the `seek` and `tell` functions, and implemented `fd_seek`
and `fd_tell` in terms of the polyfill's own position.

Also, add a dedicated stderr API for writing to stderr in a way that
tolerates strings that aren't necessarily expected to be newlines. And
add a way to test whether stderr is a terminal.

* Implement the host side of `poll_oneoff`.

This implements pseudo-futures and subscription functions, and adds
polling for streams.

* Implement clock subscriptions.

wasi.wit:

 - Remove the "timers" API from wasi-clocks, as it's now redundant with
   pseudo-future clock subscriptions.

 - Remove `subscribe-wall-clock`. Wall-clock timeouts were implemented by
   converting them to monotonic-clock timeouts anyway, so just make that
   explicit in the WASI API, and teach the polyfill how to convert
   wall-clock timeouts into monotonic-clock timeouts.

 - Move `subscribe-monotonic-clock` out of wasi-clocks and into wasi-poll,
   as it's closely tied to the pseudo-futures mechanism and the `poll-oneoff`
   implementation.

 - While here, fix `stream-read` and related functions to return an
   end-of-stream/file indicator.

Code changes:

 - `default_wall_clock()` and `default_monotonic_clock()` now always
   create a new table entry, rather than holding a table index in the
   `WasiCtx` which could potentially dangle.

 - Add support for monotonic-clock poll subscriptions.

 - Say "wall clock" instead of "system clock" when we have a choice.

* Remove the `OFlags::APPEND` flag, which is no longer used.