Function generate 

pub(crate) fn generate(
    shape: Shape,
    func: &ItemFn,
    mods: Modifiers,
    struct_ty: Option<Path>,
    attr: &TokenStream,
) -> Result<TokenStream>

Generates the trait impl for one #[sqlfunc]-annotated function, together with the items expand wraps around it.

struct_ty names the generated struct when the call site spells one.