Despite meson officially "supporting" Rust it does support working with
Cargo (Rust package manager and build system) so we need to have
a bit of custom system for using it. `cargo-output.py` to build and copy
the output files (static archive and depfile) to the expected location
by meson. And `cargo-rustc-static-libs.py` to get the common linker args
for a bar-bone std rust program (ie without dependencies).

Those two scripts would potentially be removed if meson and/or cargo
became more flexible and/or intelligent.