0.4.0

#69 Add the ability to override the visibility of generated builder.

#[derive(buildstructor::Builder)]
pub struct MyStruct {
    simple: usize,
}

The generated constructor will have private visibility and the builder will be public.

#70 Add the ability to override the visibility of generated builder.

#3 Changed the storage of required but not yet supplied values to use MaybeUninit.
The tuple that represents the builder state stays the same memory layout regardless of if a field has been initialized or not.

This does not require any unsafe code!