Update and rebase on WASI main.

As part of the move to modularize WASI, I've now reset the `main` branch
in this repository to be the same as that if the [`WASI` repository],
and this is now a PR on top of that.

This implements the directory structure described in WebAssembly/WASI#360,
with a standard directory, and a wasi-clocks directory under that.

[`WASI` repository]: https://github.com/WebAssembly/WASI/

phases/ephemeral/docs.md

@@ -1098,12 +1098,17 @@ The type of the event to which to subscribe, and the contents of the subscriptio
 
 Offset: 8
 
-## <a href="#exitcode" name="exitcode"></a> `exitcode`: `u32`
-Exit code generated by a process when exiting.
+## <a href="#exitcode" name="exitcode"></a> `exitcode`: `u8`
+Exit code generated by a program when exiting.
 
-Size: 4
+Size: 1
 
-Alignment: 4
+Alignment: 1
+
+### Constants
+- <a href="#exitcode.success" name="exitcode.success"></a> `success`
+
+- <a href="#exitcode.failure" name="exitcode.failure"></a> `failure`
 
 ## <a href="#riflags" name="riflags"></a> `riflags`: `Record`
 Flags provided to `sock_recv`.