Add Condition to `Eio.Std`

[anmonteiro]

• If I'm opening Eio.Std I'm most likely buying in to the Eio abstractions
• Just like I expected to use Promise or Switch, I usually start typing Condition.* only to find out it's the Stdlib Condition module

I propose adding the Condition module to Eio.Std to avoid the ambiguity.

[talex5]

I'm not very keen on that, since it does indeed shadow the stdlib module, which might be confusing.

[SGrondin]

At the moment I don't often open Eio.Std because I have to type "Eio." for ~90% of the Eio function calls I make anyway (Eio.Flow.*, Eio.Stream.*, Eio.Path.*, etc).

open Eio.Std is most useful for traceln, but during development I'm continuously adding and removing traceln calls while debugging, and since it's often the only usage of Eio.Std, it leads to intermittent and distracting warnings about an unused open.

I might change my workflow and start using open! instead.

To be clear, I'm not advocating for a change of direction in Eio.Std. I definitely see the advantages in the current direction. But I thought I should share this (minor) annoyance I deal with everyday in case it's useful to you all!

[talex5]

One other possibility: you can add (flags (:standard -open Eio.Std)) in your dune file to open it for all modules. I only don't do that because it might confuse people copying my code as an example.