Implement soft timeout option

[augustepoiroux]

Extend all commands with a timeout attribute.
When the timeout is reached, the REPL attempts to cancel the command and returns a timeout error. Interaction with the REPL can be resumed with new commands.

[kim-em]

Let's check that the test suite passes with the default timeout set to some long value.

[Kripner]

This feature is critical for automated theorem proving. Is there anything blocking the merge?
The test outputs are expired, so I can't see them now, but I tested the timeout logic and it seems to work.

[augustepoiroux]

Hey @Kripner!
I rebased the code on the latest main changes.
From what I remember, I was facing the following issues:

• I am using a task with IO.sleep for timeout measurement. It seems that they are not cancelled until IO.sleep terminates. This has the side effect of accumulating memory overhead when sending lots of requests in a short amount of time.
• The long_kernel_check.in test does not pass, i.e. my implementation of timeout is not working in that case.

I don't have enough bandwidth to work on this at the moment, but if you want to look into that, feel free to update this PR!