signature_derive: adds support for lifetimes

[baloo]

This allows to writes things like:

#[derive(Debug, Signer)]
pub struct EcSigner<'ctx, C>
where
    C: PrimeCurve + CurveArithmetic,
{
    context: Mutex<&'ctx mut TransientKeyContext>,
}

Usecase: parallaxsecond/rust-tss-esapi#537

[tarcieri]

Why'd you close this?

[baloo]

because I realized it was easy enough to implement manually.