Move all crates to workspace root

[epilys]

Summary of the PR

Having all the workspace crates under the crates/ directory is unnecessary. Rust documentation itself recommends all crates to be in the root directory:

https://doc.rust-lang.org/book/ch14-03-cargo-workspaces.html#creating-the-second-package-in-the-workspace

I paste the text content here, in case the online page ever changes or becomes unavailable:

## Creating the Second Package in the Workspace

Next, let’s create another member package in the workspace and call it add_one. Change the top-level Cargo.toml to specify the add_one path in the members list:

Filename: Cargo.toml

[workspace]

members = [
    "adder",
    "add_one",
]

Then generate a new library crate named add_one:

$ cargo new add_one --lib
     Created library `add_one` package

Your add directory should now have these directories and files:

├── Cargo.lock
├── Cargo.toml
├── add_one
│   ├── Cargo.toml
│   └── src
│       └── lib.rs
├── adder
│   ├── Cargo.toml
│   └── src
│       └── main.rs
└── target

Please summarize here why the changes in this PR are needed.

Requirements

Before submitting your PR, please make sure you addressed the following
requirements:

• All commits in this PR are signed (with git commit -s), and the commit
  message has max 60 characters for the summary and max 75 characters for each
  description line.
• All added/changed functionality has a corresponding unit/integration
  test.
• All added/changed public-facing functionality has entries in the "Upcoming
  Release" section of CHANGELOG.md (if no such section exists, please create one).
• Any newly added unsafe code is properly documented.

[sboeuf]

That sounds good to me. Getting rid of the crates indirection directory is a nice simplification.

[epilys]

@liuw @sameo @jiangliu ping :)

[epilys]

This is approved, needs someone to press the merge button.

[epilys]

Rebased against main