Fix WASI struct sizes

[dcodeIO]

fixes #1116 if it turns out that the new assumptions made there are correct. Also see the linked WASI issue.

[MaxGraey]

@dcodeIO I'm wondering what if introduce @alignas decorator for such cases?

@unmanaged @alignas(48)
export abstract class event {
   /** User-provided value that got attached to `subscription#userdata`. */
  userdata: userdata;
  /** If non-zero, an error that occurred while processing the subscription request. */
  error: errno;
  /** The type of the event that occurred. */
  type: eventtype;
}

wdyt?

[dcodeIO]

Seems useful in certain use cases, but might also have some strange edge cases here. For example, what do we do if an extension of the class doesn't fit into the base class's alignment? Do we silently double the size, or should there be an error. Also we'd probably still have some __padding fields, for example when modeling unions or variable-size fields, i.e. usize.

[MaxGraey]

C++ for example just ignore alignment with less value than in base class for derived classes:
https://repl.it/repls/IdealEnlightenedZettabyte