Add generics and lifetimes on element macro

[supersurviveur]

Fixes #15.

This PR adds lifetimes, generics and where clauses on the struct definition and trait implementation created with the zyn::element proc-macro attribute.

This allows doing the following code :

#[zyn::element]
fn generics<T: zyn::ToTokens>(children: T) -> zyn::TokenStream {
    zyn::quote::quote!(#children)
}
#[zyn::element]
fn lifetimes<'a>(ident: &'a zyn::syn::Ident) -> zyn::TokenStream {
    zyn::zyn!(struct {{ ident }};)
}

[aacebo]

sorry for missing this, and thanks for the contribution!