Experimental bindings to GTK 4.0 for OCaml.

Generates direct OCaml C bindings using GObject introspection data

• No enumeration / constants mapping
• No inline docs
• Currently, impact on binary size is significant: example0.exe, a trivial hello-world example, is 4.7M on my Macbook. It is not yet clear whether it will be feasible to improve this significantly. See #11.

• Requires gtk-4 with its development files installed, and the GObject Introspection utilities (g-ir-generate).
• Python 3 (used to convert the GObject introspection files to ML and C code)
• OCaml: opam switch with dune installed

On Ubuntu 22.04.1, these system packages are required:

• libgtk-4-dev
• gobject-introspection
• gir1.2-glib-2.0
• gir1.2-gtk-4.0
• python3-minimal
• libpython3.10-stdlib

1. Set the PREFIX variable to the location where girepository-1.0 typelibs are installed
   • if using Ubuntu packages on x86-64, this is /usr/lib/x86_64-linux-gnu
   • if using Linux Brew, this is /home/linuxbrew/.linuxbrew/lib
2. Run dune build
3. Built example programme can be found at _build/default/examples/example0.exe

We intend for the following safety property to hold:

Assuming the GObject Introspection data is accurate, an OCaml program that uses the generated bindings (and that does not use unsafe OCaml features such as Obj.magic) shall not exhibit undefined behavior, except that:

• record allocation functions such as Gtk.TextIter_.alloc_uninit_UNSAFE do not initialize the record's fields; passing such uninitialized records to APIs that expect the record to be initialized is unsafe
• textBuffer#get_start_iter and similar APIs do not increment the TextBuffer object's refcount; i.e. a TextIter does not own a reference to a TextBuffer. So the existence of a TextIter i does not keep i#get_buffer alive. However, if GNOME/gtk!6087 is merged, using a TextIter after the TextBuffer has been freed will with very large probability either cause a clean segfault or cause an "Invalid text buffer iterator" warning to be printed. (Using i#get_buffer in other ways, however, has undefined behavior.)
• Similar issues probably exist with other types of records.

Known violations of this property should be recorded as issues with label safety.