WIP fluent generator

[keynmol]

Closes #6

Goal

Provide idiomatic Scala bindings using regular classes, methods, traits, etc. to recover the inherently object oriented nature of gnome libraries. These bindings are called fluent.

The aim is to eliminate all mentions of scalanative.unsafe types from the bindings, by means of copying, allocations, and other things that'd make a C developer's hair stand up. As such, these bindings introduce overhead, but in most cases this should be fine. Getting unsafe backing values from fluent wrappers should be easy, to use those on the hotpath and delegate straight to C bindings.

The raw C bindings are not deprecated, just moved to internal package. They power fluent bindings, and will always be shipped in same artifact.

The code is in fluent-generator module, and you can give it a jolly good whirl by running gobject/generateFluentBindings.

At this point the goal of this PR is to

TODO:

• throws=1 on methods manifests itself as a last parameter error: GError**, we should produce it as Either[Error, A].
  • read semantics of https://docs.gtk.org/glib/error-reporting.html
  • idiomatic record wrappers? Can we completely eliminate scalanative.unsafe types from the fluent API? How would that work?
• nullable=1 on methods - should produce Option[A] as a result. May be check if that exists on parameters as well?
• Arrays - not handled well at all. Should provide a union type option with Zone (like String | CString)
• Virtual methods? Whatever they are
• Ownership? Does it even make sense?
• Traits being returned from methods - Perhaps we need an anonymous class. I think that's what will eventually make use of virtual methods - they're part of the interface I think
• Render docs?
• Out parameters? They have direction="out". We should change the output of a function to be a tuple with all those parameters
• function - static methods on companion objects
• <varargs>