chore: put everything in One Big Workspace

This commit changes the Cargo workspace setup to put all crates in One
Big Workspace, rather than having separate workspaces for some targets.
We now use the `per-package-target` unstable cargo feature to build
different crates for different targets. This means that `cargo` commands
in the root workspace now work without requiring the user to `cd` into a
particular directory to build a platform target --- for example, I can
now run:

```console
# in the repo root directory
$ cargo build -p mnemos-d1 --bin mq-pro
```

and build a MnemOS binary for the MQ Pro, without having to `cd` into
the MQ Pro directory.

One issue is that `cargo build --workspace` (and `check --workspace`,
etc) still does not work correctly, due to some [weird][1] [issues][2]
with feature unification which I don't entirely understand. However, as
a workaround, I've changed the workspace Cargo.toml to add a
[`default-members` field][3], so that running `cargo build` or `cargo
check` _without_ `--workspace` will build the subset of crates in the
`default-members` key. This way, `cargo {build, check, etc}` in the repo
root will do something reasonable by default, and the actual platform
targets can be built/checked with `cargo $WHATEVER --package $CRATE`.
IMO, this is still substantially nicer than having a bunch of separate
workspaces.

[1]: ia0/data-encoding#47
[2]: bincode-org/bincode#556
[3]: https://doc.rust-lang.org/cargo/reference/workspaces.html#the-default-members-field

Author: hawkw

.cargo/config.toml

@@ -1,3 +1,12 @@
+[alias]
+crowtty = "run --bin crowtty --release --"
+melpomene = "run --bin melpomene --release --"
+melpo = "melpomene"
+forth3 = "run --bin f3repl --release --"
+
 [build]
 # Currently needed for `tokio-console` support.
-rustflags = ["--cfg", "tokio_unstable"]
+rustflags = ["--cfg", "tokio_unstable"]
+
+[target.riscv32imac-unknown-none-elf]
+runner = "espflash flash --monitor"