Timers

[mxgrey]

This PR splits the timer.rs module out of #446 so it can be reviewed separately after #446 is merged.

[mxgrey]

@esteve This is embarrassing since I was vocally supportive of committing the cargo lockfiles, but now I'm thinking that might have been a mistake. I'm noticing two issues in practice:

1. Every run of colcon build seems to update the lockfiles. In fact a bunch of different tools that I use on a regular basis are triggering automatic updates of the lockfiles. It's creating a lot of unnecessary churn in the git history.
2. More crucially, I'm seeing this error happen in the minimum supported Rust compiler version CI. Essentially any new lockfiles that get generated and committed will be incompatible with the "Minimal" CI.

I've had to resort to manually deleting the Cargo.lock for the worker_demo, logging_demo, and now timer_demo examples each time I commit in order to get the Minimal CI to pass.

I'd suggest we remove the lockfiles from the git history or else this is going to cause a great deal of ongoing annoyance where everyone is constantly reverting lockfiles before committing 😞

[mxgrey]

@knmcguire Sorry to summon you into this random PR but it seems a new CI failure has popped up for Windows.

I have a feeling this is due to this recent PR in colcon-cargo, and we'll see this failure show up for the main branch on Tuesday during its weekly CI run.

[knmcguire]

Thanks for letting me know @esteve ! I don't get messages of the CI failures on windows so I was unaware of this. I'll dig into it and make a PR

The cargo lock did solve some things for the windows CI though so if that is removed we might need to deal with those failures, but let's see when it comes.

[knmcguire]

See this PR: #495