fix: support non-receiver lifetimes in trait functions

[matejcik]

if a trait function has lifetimes other than the receiver, codegen would botch the signature.

either of the following wouldn't work:

#[stabby::stabby]
pub trait MyTrait {
    fn foo<'a>(&self, arg: &'a u8) -> &'a u8;
    fn bar<'a, 'b>(&'a self, arg: &'b u8) -> &'a u8;
}

this PR fixes the problem by:

• making sure to copy over all lifetimes from the original signature, and
• only if the receiver is unnamed in the original signature, creating a separate entry for it

[p-avital]

That looks lovely, thanks for fixing that oversight! :)