Use . instead of / for interface members.

Say:
```
wasi:io/streams.stream-error
```
instead of:
```
wasi:io/streams/stream-error
```

This better aligns with WIT's `use` syntax and the upcoming nested
namespaces feature in WIT.

Author: sunfishcode

wit/types.wit

@@ -110,8 +110,8 @@ interface types {
   /// provided.
   ///
   /// Stream operations which return
-  /// `wasi:io/stream/stream-error::last-operation-failed` have a payload of
-  /// type `wasi:io/error/error` with more information about the operation
+  /// `wasi:io/stream.stream-error.last-operation-failed` have a payload of
+  /// type `wasi:io/error.error` with more information about the operation
   /// that failed. This payload can be passed through to this function to see
   /// if there's http-related information about the error to return.
   ///