Convert to slog as our logging library

[mkeeter]

Per RFD 659, this converts us to using slog everywhere.

This PR is mostly mechanical! slog expects a logger object to be threaded through function calls. Here, we lazily construct it in the Cli, then grab it from the ExecutionContext. It's mostly passed as an argument, although in places where we build an XyzContext (e.g. HiffyContext), we clone a copy of the logger and store it locally to reduce boilerplate.

(cloning a slog::Logger is expected to be cheap)

Note that we still use env_logger for configuration in the humility-bin package, so that we can get verbose probe-rs logs (and maybe other libraries as well, but that's the loudest one).

[mkeeter]

One thing that I'd like opinions on: should we re-export the slog types from humility (perhaps via the now-deleted humility-log crate)? I could imagine exporting the various logging macros and Logger type, so that people using downstream crates wouldn't need a slog dependency of their own.

[mkeeter]

One thing that I'd like opinions on: should we re-export the slog types from humility (perhaps via the now-deleted humility-log crate)? I could imagine exporting the various logging macros and Logger type, so that people using downstream crates wouldn't need a slog dependency of their own.

I've decided that I like this, but it's in a standalone commit at the top of the stack if anyone strongly disagrees.