Draft: Port to meson and cargo-c

This is a work in progress. Current status:

• Build and install library
• Build and install rsvg-convert
• Hook to Rust tests
• Generate rsvg-version.h
• Install header files
• Install .pc file
• Hook to C API tests
• GIR - GObject Introspection and typelib
• Build vala VAPI - see libgweather/libgweather/meson.build (depends on GIR)
• Build docs with gi-docgen (depends on GIR)
• Build and install rsvg-convert man page
• gdk-pixbuf loader - see if !722 (merged) makes this easier
• Change the CI to include meson and cargo-c in the images
• Fix the msys2 job in the CI - fixed in !728 (merged)
• Do we need a distcheck job in the CI?
• Fix the reference and pages jobs in the CI (depends on GIR)
• Notify the release team and gnome-build-meta
• Notify downstreams; let them find packaging problems

Problems, to solve later:

• Too much rebuilding? The library gets built three times: cargo cbuild for the C library, "cargo build" for rsvg-convert, and cargo test for the tests.
• My meson code is messy, clean it up plz.

This will fix #552 (closed).