Initial transpile of dav1d

[randomPoison]

Transpile libdav1d and the dav1d CLI tool to Rust using c2rust. This initial version can disable features like assembly as needed to get an initial Rust translation of the library that builds.

[randomPoison]

I've pushed an initial fork of dav1d that includes transpiled Rust sources alongside their original C equivalents in 099f2b2. To get it to work, I needed to do a few things:

• Build and install c2rust from WIP: Support C11 atomics required by dav1d immunant/c2rust#814 to work around a bug that was causing the transpile to fail.
• Configure meson with the build options enable_asm=false and bitdepths=['16'].
• Specifically build the tools/dav1d ninja target when generating compile_commands.json to avoid transpiling the files in tests/checkasm because c2rust cannot currently translate those files.

The full process for doing the transpile is:

mkdir build && cd build
meson setup ..
meson configure -Denable_asm=false "-Dbitdepths=['16']"
cd ..
bear -- ninja -C build tools/dav1d
c2rust transpile compile_commands.json -b dav1d --overwrite-existing

[kkysen]

Btw, meson configure should generate a compile_commands.json on its own, so we wouldn't have to use bear and compile. Though if we want to skip compile commands that don't generate dav1d (e.x. tests/checkasm), we might have to do it this way anyways. Does this way still transpile any other non-checkasm tests?

Also, there's already a Rust port of checkasm, rust-av/checkasm-rs, so we should try to use that.

[randomPoison]

Doing bear -- ninja -C build tools/dav1d doesn't compile any of the tests, so currently none of the tests have been transpiled. That was the main reason for using bear instead of the compile_commands.json generated by meson configure.