fix(rs-port): wrong implemenetation for user data in MetacallFuture

[hulxv]

Description

Please include a summary of the change and which issue is fixed. List any dependencies that are required for this change.

Fixes #(issue_no)

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[hulxv]

@devraymondsh @viferga

[devraymondsh]

The X_untyped function format is designed so that the user is not forced to pass a generic and would use the value through pointer polymorphism. Adding a generic to it would not only be a noticeable breaking change but I wouldn't think it's a good idea anyway. I suggest making a new API specifically for async functions which would be something like X_async or something similar.