Relax return types of Eio.Process.pipe

[Arogbonlo]

This updates the pipe function in the Make_mgr module to use polymorphic variants for its return types. The change allows the returned source and sink to be compatible with APIs expecting monomorphic types. Change:

• Modified pipe function signature in Make_mgr
• Updated from [...] to [>...] for both source and sink types

This addresses issue #750, improving flexibility when working with various APIs that use stricter type definitions for sources and sinks.

• Added new test case demonstrating compatibility with constrained types

[patricoferris]

Hey @Arogbonlo,

Thanks for the PR. There are a few issues here that need addressed:

• The original issue was to make the return type of pipe allow for all possible subtypes (<) not to have at least the variants it describes (>).
• This is a type change and doesn't really require a test.

I think perhaps you should start again updating the types in lib_eio/process.{ml.mli} and following this through to eio_unix. I think you are also going to need to update the Pipe effect in lib_eio/unix/private.ml too. No other parts of the types should be changed (e.g. you changed _ mgr to 'a t)

[Arogbonlo]

I'll effect them immediately. Thank you. @patricoferris

[patricoferris]

Hey @Arogbonlo how's it going? Do you need any help?

[Arogbonlo]

Hey @Arogbonlo how's it going? Do you need any help?

Thank you so much for the check!

I'll push my progress in a few hours, so you can check it and let me know where i have to make changes.

[Arogbonlo]

Hey @patricoferris

I just made a commit, please I'll appreciate your feedback on it.

Thank you.