Add Rust code to the build

Just to get the build going with some Rust code in it:

• Update the CI a bit
• Include my script from gnome-shell to call cargo from meson.
• From the C code, call a ipuz_noop() implemented in Rust.
• Generate the ipuz-rust.h header instead of writing it by hand.
• Discuss how to call C code from Rust - use bindgen, or write prototypes individually as we need them?
• Fix the CI
• Change the code coverage job so it works with a mixed C+Rust code base.

C header file from the Rust code

I plan to use cbindgen; it works really well. For this initial build, I'm just using a hand-written ipuz-rust.h file. The idea is to generate that completely with cbindgen.

Calling C code from the Rust code

Writing C function prototypes on the Rust side is... not hard? It's what we did in librsvg since we were adding them piecemeal, as we needed them.

For large-scale automated C-to-Rust stuff, there is bindgen. It may help with translating data structures. I haven't used it personally; it generates a lot of code and fancy tests, but it works.