chore: put everything in one big Cargo workspace

[hawkw]

Currently, this repo contains three separate Cargo workspaces: one for
source/, which contains the actual mnemOS crates, and then a separate
root workspace for each crate in tools/.

This is a bit unfortunate, as it means that it's not possible to run
cargo run --bin WHATEVER from the root directory of the repo. Instead,
in order to run melpomene, it's necessary to cd into source.
Similarly, if I want to run tools like crowtty, I need to cd into
tools/crowtty. This is very mildly annoying. The lack of a global
workspace also means that it's not currently possible to run cargo
commands like cargo build --workspace and build everything in one go.

This branch rectifies this by moving the workspace's Cargo.toml to the
root of the repo, and including all crates in the workspace (both
source/ and tools/). The potential downside of this is that it means
there's a lockfile, Cargo.toml, and .cargo/ directory in the root of
the workspace, which previously was nice and clean, and didn't have a
bunch of random config files and stuff in it. But, I think this is
probably not that bad?

If you'd really prefer not to make this change, that's fine --- having
to change dirs was really only a minor inconvenience.

[jamesmunns]

My only concern with this is that I have some memory of workspaces for multiple targets (e.g. desktop linux + cortex-m) not working right/being a pain in the ass.

I can't remember if it was cargo or rust-analyzer's fault, but the original division of workspaces was made to avoid "embedded" crates and "desktop crates" being in the same workspace.

That being said - in this consideration melpo should probably be in the "tools" folder.

I'm happy to merge this now, but it might make sense to back out later when we get closer to working with real hardware.

[hawkw]

My only concert with this is that I have some memory of workspaces for multiple targets not working right/being a pain in the ass.

I can't remember if it was cargo or rust-analyzer's fault, but the original division of workspaces was made to avoid "embedded" crates and "desktop crates" being in the same workspace.

That being said - in this consideration melpo should probably be in the "tools" folder.

I'm happy to merge this now, but it might make sense to back out later when we get closer to working with real hardware.

Ah, hmm, I was wondering if there was some issue like that. I've not had problems with having all the mycelium stuff in the same workspace even though it will do builds for both the host target (build tools) and the actual OS target (which is still x86 in my case but uses a very different target.json). But, it's possible that only works because of some specific aspect of the way I've set things up.

If this causes problems, we can definitely undo it later.