Debugging proc macros is very difficult as we can't easily get
backtraces into the proc macro code. Improved error messages will
hopefully give some hint to the user on what the problem is.

    error: inconsistent struct constructor
        --> src/item.rs:1497:16
         |
    1497 |               Ok(Signature {
         |  ________________^
    1498 | |                 constness,
    1499 | |                 asyncness,
    1500 | |                 unsafety,
    ...    |
    1508 | |                 generics,
    1509 | |             })
         | |_____________^ help: try: `Signature { constness, asyncness, unsafety, abi, fn_token, ident, generics, paren_token, inputs, variadic, output }`
         |
         = note: `-D clippy::inconsistent-struct-constructor` implied by `-D clippy::all`
         = help: for further information visit https://rust-lang.github.io/rust-clippy/master/index.html#inconsistent_struct_constructor

rust-lang/rust-clippy#6795

    error: manual implementation of `Option::map`
        --> src/item.rs:1194:22
         |
    1194 |               let ty = if let Some(eq_token) = input.parse()? {
         |  ______________________^
    1195 | |                 Some((eq_token, input.parse::<Type>()?))
    1196 | |             } else {
    1197 | |                 None
    1198 | |             };
         | |_____________^ help: try this: `input.parse()?.map(|eq_token| (eq_token, input.parse::<Type>()?))`
         |
         = note: `-D clippy::manual-map` implied by `-D clippy::all`
         = help: for further information visit https://rust-lang.github.io/rust-clippy/master/index.html#manual_map

Replace assertions in Puctuated with more helpful panic messages

This makes the complete error message more easily greppable.

Add Lifetime span getter and setter