Add console-proxy command

[jgallagher]

humility console-proxy uses the uart control functions added in oxidecomputer/hubris#1005 to do its best (very slow) picocom impersonation.

Before merging:

• don't try to build this command (or make it report "this isn't supported") on Windows
• don't allow this command to be used within the repl on non-Windows

[mkeeter]

Is there a way to make this a conditional workspace dependency?

[jgallagher]

Maaaaybe? I don't think we can have target-specific workspace dependencies (e.g., rust-lang/cargo#5220), but maybe we can specify termios = "0.3" as a workspace dep and conditionally depend on it in Cargo.toml. Trying that in 5db1630 to see if it passes Windows CI.

[jgallagher]

Looks like that worked fine. 👍

[steveklabnik]

I had tried doing this in the workspace's cargo.toml and it didn't seem to work. That is, literally this diff, not what @jgallagher just tried. it might!

... is the policy that every single thing becomes a workspace dependency now? I'd personally default to only things that are shared, but I can see wanting to do it all there, too.

[jgallagher]

I'd lean toward making them all workspace deps, because otherwise if I want to add a new dep, I first ought to grep and see if it's used by something else, and if so, move it to be a workspace dep now that it's becoming shared.

That's slightly annoying but not the end of the world; are there similar annoyances with "everything at the workspace level" I'm not thinking of?