bindgen-cli: depend on libclang-dev, recommend rustfmt and fix long description

TL;DR: bindgen MUST depend on libclang1, SHOULD recommend rustfmt, and it'd better depend on libclang-dev

Since #1070241 has been the only open bug on my DDPO for a while, I thought I should give it a shot :-)

In bug #1070241 the reporter raises the issue that bindgen cannot work without clang. To show this, they report the following error message:

/usr/include/zconf.h: 254 fatal error 'stddef.h' not found

This error is triggered by the absence of /usr/lib/llvm-16/lib/clang/16/include/stddef.h (not just any other stddef.h, I checked this) on the system, which is provided by libclang-common-16-dev (of course the llvm version is not important here).

Now, stddef.h is not actually strictly needed for bindgen to function. I tested in a minbase unstable chroot that, for a minimal header.h like

struct s {
  int integer;
};

all bindgen needs is libclang1. Without the latter, it throws out the following panic:

root@debian:~# bindgen header.h 
thread 'main' panicked at /usr/share/cargo/registry/bindgen-0.66.1/lib.rs:604:31:
Unable to find libclang: "couldn't find any valid shared libraries matching: ['libclang.so', 'libclang-*.so', 'libclang.so.*', 'libclang-*.so.*'], set the `LIBCLANG_PATH` environment variable to a path where one of these files can be found (invalid: [])"
note: run with `RUST_BACKTRACE=1` environment variable to display a backtrace

After installing libclang1 (with no recommends), on the other hand, the bindings are generated, but with a warning:

Failed to run rustfmt: cannot find binary path (non-fatal, continuing)

Thus bindgen MUST depend on libclang1, and SHOULD at least recommend rustfmt (without it the generated bindings look like a mess. They will work but they are unreadable).

As for stddef.h, that's needed by libclang so that bindgen can generate bindings for some (not all) headers, namely those which expect stddef.h to be available, like zlib's zconf.h. For instance, again in a minbase unstable chroot,

root@debian:~# apt install --no-install-recommends bindgen libclang1 zlib1g-dev
root@debian:~# bindgen /usr/include/zconf.h 
/usr/include/zconf.h:254:14: fatal error: 'stddef.h' file not found
panicked at main.rs:52:36:
Unable to generate bindings: ClangDiagnostic("/usr/include/zconf.h:254:14: fatal error: 'stddef.h' file not found\n")

which is the error reported in the bug. I'd argue that bindgen should not fail because of missing clang headers. This means libclang-common-$VERSION-dev SHOULD be a dependency of bindgen as well.

Finally, to minimize the risk of other code requiring other libclang/libgcc/libstdc++ headers for bindings generation, I'd just make bindgen depend on the whole libclang-dev, which also pulls in libclang1, libclang-common-$VERSION-dev, and the gcc/c++ headers. After all, bindgen's requirement page (https://rust-lang.github.io/rust-bindgen/requirements.html) would have us also install llvm-dev and clang itself (which may be overshoot in the absence of reports that those dependencies are actually needed), so only installing libclang-dev should not be too much.