Now that `syn` and `quote` are available in the Kernel use them for the
`module!` proc-macro instead of the current string based proc-macro
approach.

Proc-macros written using `syn` are a lot more readable and thus easier
to maintain than proc-macros written using string manipulation. An
additional advantage of `syn` is the improved error reporting, here is
an example:

    module! {
        type: RustMinimal,
        name: "rust_minimal",
        author: "Rust for Linux Contributors",
        description: "Rust minimal sample",
        // we forgot to add a `license` field.
    }

Prior to this patch, the error reads:

    error: proc macro panicked
      --> samples/rust/rust_minimal.rs:7:1
       |
    7  | / module! {
    8  | |     type: RustMinimal,
    9  | |     name: "rust_minimal",
    10 | |     author: "Rust for Linux Contributors",
    11 | |     description: "Rust minimal sample",
    12 | |     // license: "GPL",
    13 | | }
       | |_^
       |
       = help: message: Missing required key "license".

    error[E0425]: cannot find value `__LOG_PREFIX` in the crate root
      --> samples/rust/rust_minimal.rs:21:9
       |
    21 |         pr_info!("Rust minimal sample (init)\n");
       |         ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^ not found in the crate root
       |
       = note: this error originates in the macro `$crate::print_macro` which comes from the expansion of the macro `pr_info` (in Nightly builds, run with -Z macro-backtrace for more info)

    error[E0425]: cannot find value `__LOG_PREFIX` in the crate root
      --> samples/rust/rust_minimal.rs:22:9
       |
    22 |         pr_info!("Am I built-in? {}\n", !cfg!(MODULE));
       |         ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^ not found in the crate root
       |
       = note: this error originates in the macro `$crate::print_macro` which comes from the expansion of the macro `pr_info` (in Nightly builds, run with -Z macro-backtrace for more info)

    error[E0425]: cannot find value `__LOG_PREFIX` in the crate root
      --> samples/rust/rust_minimal.rs:35:9
       |
    35 |         pr_info!("My numbers are {:?}\n", self.numbers);
       |         ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^ not found in the crate root
       |
       = note: this error originates in the macro `$crate::print_macro` which comes from the expansion of the macro `pr_info` (in Nightly builds, run with -Z macro-backtrace for more info)

    error[E0425]: cannot find value `__LOG_PREFIX` in the crate root
      --> samples/rust/rust_minimal.rs:36:9
       |
    36 |         pr_info!("Rust minimal sample (exit)\n");
       |         ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^ not found in the crate root
       |
       = note: this error originates in the macro `$crate::print_macro` which comes from the expansion of the macro `pr_info` (in Nightly builds, run with -Z macro-backtrace for more info)

    error: aborting due to 5 previous errors

After this patch, the error reads:

    error: Missing required key "license".
      --> samples/rust/rust_minimal.rs:8:5
       |
    8  | /     type: RustMinimal,
    9  | |     name: "rust_minimal",
    10 | |     author: "Rust for Linux Contributors",
    11 | |     description: "Rust minimal sample",
       | |_______________________________________^

    error: aborting due to 1 previous error

Similar improvements in error quality apply to getting the order of the
fields wrong and giving non-ascii strings as values for fields where it
should be ascii-only.

Signed-off-by: Benno Lossin <[email protected]>