Add Eio.Pool.use ~never_block

[SGrondin]

This PR adds the optional parameter ?never_block:bool to Eio.Pool.use. The default is false.

Quoting pool.mli:

If true and the pool has reached maximum capacity, then a fresh resource is created to ensure that this use call does not wait for a resource to become available. This resource is immediately disposed after [f x] returns.

In my opinion, this feature makes Eio.Pool a valid choice for a wider range of use cases.

Method	Availability	Overhead
Creating a resource every time	✅ Never blocks	🔴 No reuse
Eio.Pool.use	🔴 Can block	✅ Maximal reuse
Eio.Pool.use ~never_block:true	✅ Never blocks	🟡 Some reuse