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.

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`.