Add support for generic types

[turion]

Closes #424. Allows generic type parameters in the derive macros:

    #[derive(NifStruct)]
    #[module = "GenericStruct"]
    pub struct GenericStruct<T> {
        t: T
    }

[turion]

CC @evnu this obsoletes #425

[turion]

Note that functions to which the #[rustler::nif] macro is applied must still be monomorphic, otherwise rustler wouldn't know into which type to decode.

[turion]

I'm not sure where I should add documentation.

[evnu]

Thanks, good work! Some comments from my side.

[evnu]

LGTM.