add generics support

[Jezza]

Adds support for generic enums and structs.

So the example from the doc could work like this:

    #[derive(Debug, PartialEq)]
    #[revision::revisioned(revision = 3)]
    pub enum TestEnum<T> {
        #[revision(end = 2, convert_fn = "upgrade_zero")]
        Zero,
        #[revision(end = 2, convert_fn = "upgrade_one")]
        One(T),
        #[revision(start = 2)]
        Two(u64),
        #[revision(start = 2)]
        Three {
            a: i64,
            #[revision(end = 2, convert_fn = "upgrade_three_b")]
            b: f32,
            #[revision(start = 2)]
            c: u32,
            #[revision(start = 3)]
            d: T
        },
    }

Probably not the cleanest code, as I was about halfway writing it when I realised I don't need that functionality.
but I thought I might as well get it working so you could choose to take it or not.

I'm happy to adjust any smaller things.