Initial Rust support

Currently we have several lines of work:

  - Integrating with the kernel tree and build system (Nick's & mine,
    both uploaded as branches, based on `rustc`; and another one I have
    been working on, based on `cargo`).

  - Bindings and the first bits of functionality (Alex's & Geoffrey's,
    based on `cargo`).

This patch effectively merges the work we have been doing and
integrates it in the latest mainline kernel tree.

This does *not* mean anything needs to stay as-is, but this gives us
a working, common base to work and experiment upon. Hopefully, it will
also attract external people to join!

As a summary, I added:

  - `cargo` integration with the kernel `Makefile`s:
      + Virtual `cargo` workspace to have a single lock file and to share
        deps between `cargo` jobs.
      + Picks the same optimization level as configured for C.
      + Verbose output on `V=1`.
      + A `cargoclean` target to clean all the Rust-related artifacts.

  - Initial support for built-in modules (i.e. `Y` as well as `M`):
      + It is a hack, we need to implement a few things (see the `TODO`s),
        but it is enough to start playing with things that depend on `MODULE`.
      + Passes `--cfg module` to built-in modules to be able to compile
        conditionally inside Rust.
      + Increased `KSYM_NAME_LEN` length to avoid warnings due to Rust
        long mangled symbols.

  - Rust infrastructure in a new top level folder `rust/`:
      + A `kernel` package which contains the sources from Alex &
        Geoffrey's work plus some changes:
          * Adapted `build.rs`.
          * Removed the `THIS_MODULE` emulation until it is implemented.
          * Removed `Makefile` logic and the code that `cfg`-depended
            on kernel version (no need in mainline).
          * Moved the helpers to be triggered via normal compilation,
            renamed them under `rust_*` and exported via
            `EXPORT_SYMBOL` instead.
          * Added a prelude.
      + A `shlex` package which serves as an example of an "inline"
        dependency (i.e. package checked out copy to avoid the network)

  - The example driver was setup at `drivers/char/rust_example/`.

  - Misc
      + The beginning of `Documentation/rust/` with a quick start guide.
      + `MAINTAINERS` entry.
      + SPDXs for all files.

Other notes that aren't in `TODO`s:

  - We could minimize the network requirements (use `bindgen` binary, use more
    inline dependencies...), but it is not clear it would be a good idea for
    the moment due to `core`/`alloc`/`compiler-builtins`.

  - The intention of `rust/` is to have a place to put extra dependencies
    and split the `kernel` package into several in the future if it grows.
    It could resemble the usual kernel tree structure.

  - With several drivers being built-in, `cargo` recompiles `kernel`
    and triggers duplicate symbol errors when merging thin archives.
    We need to make it only compile `kernel` once.

  - When the above works, then `make`'s `-j` calling concurrent `cargo`s
    (e.g. several drivers at the same time) should be OK, I think.
    According to rust-lang/cargo#2486  it shouldn't
    miscompile anything, but if they start locking on each other
    we will have make jobs waiting that could have been doing something else.

Signed-off-by: Miguel Ojeda <miguel.ojeda.sandonis@gmail.com>

.gitignore

@@ -152,3 +152,6 @@ x509.genkey
 
 # Clang's compilation database file
 /compile_commands.json
+
+# Rust (cargo) compilation artifacts
+/target/

Cargo.toml

@@ -0,0 +1,10 @@
+# SPDX-License-Identifier: GPL-2.0
+
+# TODO: generate automatically
+[workspace]
+members = [
+    "rust/shlex",
+    "rust/kernel",
+    "drivers/char/rust_example",
+]
+