Backport fd as handle to past and current snapshots (WebAssembly#238)

This commit backports fd as handle to past and current snapshots.
I'm hoping this could prove useful in situations where currently
in the implementor's code, any syscall that deals with `fd`s has
to be marked as potentially "unsafe", whereas with `fd` as a handle
this would be shifted to the fact that `fd` *is* a handle rather
than an alias to an int type.

phases/old/snapshot_0/docs.md

@@ -368,9 +368,10 @@ If [`rights::fd_write`](#rights.fd_write) is set, includes the right to invoke [
 - <a href="#rights.sock_shutdown" name="rights.sock_shutdown"></a> `sock_shutdown`
 The right to invoke [`sock_shutdown`](#sock_shutdown).
 
-## <a href="#fd" name="fd"></a> `fd`: `u32`
-A file descriptor index.
+## <a href="#fd" name="fd"></a> `fd`
+A file descriptor handle.
 
+### Supertypes
 ## <a href="#iovec" name="iovec"></a> `iovec`: Struct
 A region of memory for scatter/gather reads.
 

phases/old/snapshot_0/witx/typenames.witx

@@ -273,8 +273,8 @@
   )
 )
 
-;;; A file descriptor index.
-(typename $fd u32)
+;;; A file descriptor handle.
+(typename $fd (handle))
 
 ;;; A region of memory for scatter/gather reads.
 (typename $iovec

phases/snapshot/docs.md

@@ -368,9 +368,10 @@ If [`rights::fd_write`](#rights.fd_write) is set, includes the right to invoke [
 - <a href="#rights.sock_shutdown" name="rights.sock_shutdown"></a> `sock_shutdown`
 The right to invoke [`sock_shutdown`](#sock_shutdown).
 
-## <a href="#fd" name="fd"></a> `fd`: `u32`
-A file descriptor index.
+## <a href="#fd" name="fd"></a> `fd`
+A file descriptor handle.
 
+### Supertypes
 ## <a href="#iovec" name="iovec"></a> `iovec`: Struct
 A region of memory for scatter/gather reads.
 

phases/snapshot/witx/typenames.witx

@@ -273,8 +273,8 @@
   )
 )
 
-;;; A file descriptor index.
-(typename $fd u32)
+;;; A file descriptor handle.
+(typename $fd (handle))
 
 ;;; A region of memory for scatter/gather reads.
 (typename $iovec