On macOS, this was done by default regardless of what the user wants.
This was also not done on Windows or Linux. Instead, we now provide a
`--open-log` flag to open the log file in the default browser, and it
works on all platforms.
This patch updates the CLI to use anyhow and thiserror for error
management. The main feature that our custom `Error` type was providing
was a _list_ of messages, which would allow us to annotate "lower-level"
errors with more contextual information. This is exactly what's
provided by anyhow's `Context` trait.
(This is setup work for a future PR that will pull the `config` and
`loader` modules out into separate crates; by using `anyhow` we wouldn't
have to deal with a circular dependency between with the new crates.)
* Structure `cli` crate as both a library and an executable, so that
benchmarks can import code from the crate.
* Import macros in the Rust 2018 style.
