Generate mutually-exclusive types depending on its version

[diamondburned]

Right now, type generators for classes, interfaces and records all generate its methods in the same file as the declaration, which means that methods added later will be grouped into the same unversioned file.

The solution seems to be to generate one additional file containing the base version that wouldn't be included if any versioning build tags are provided. This is because we cannot define methods in addition to existing ones if we're declaring everything inside an interface, so the interface declarations will have to be mutually exclusive somehow.

An example:

gtkwidget.go

// go:build cgo && !gtk-3.32
type Widget interface {
    BaseMethod()
}

gtkwidget-3.32.go

//go:build cgo && gtk-3.32
type Widget interface {
    BaseMethod()
    NewlyAddedMethod()
}

[diamondburned]

As of right now, the generator cannot generate versioned files at all, even if the code is seemingly there. The feature isn't considered to be complete, however, so it's technically not broken.

[diamondburned]

Replaced by #24.