Xtask improvements

[egithinji]

Some minor improvements to an already-merged PR (#2688) on the task automation via xtask. Main ones being:

• Adding more explanatory comments about what the xtask package is and what it does
• Using Clap for CLI arg parsing
• Using Anyhow for error handling

[mgeisler]

I'm surprised it should be necessary with this case? I would have expected Clap to be able to handle it for us?

We can merge this PR now, but could you look into using a Rust enum to represent the available commands? I'm pretty sure Clap can be told that a command line argument must be a valid enum variant — and so it can return an error earlier if people use an unrecognized name.

I think there is an example of what I mean here: https://docs.rs/clap/latest/clap/_cookbook/cargo_example_derive/index.html

We want each xtask (such as install-tools) to become a sub-command.

[mgeisler]

Thanks @egithinji! Let's merge this now since it's a nice improvement to what we had before.