Most of the generated code should carry span info about its origins

[CreepySkeleton]

error[E0277]: the trait bound `u32: std::convert::From<&str>` is not satisfied

  --> tests/skip.rs:98:14

   |
98 |     #[derive(StructOpt, Debug, PartialEq)]
   |              ^^^^^^^^^ the trait `std::convert::From<&str>` is not implemented for `u32`

   = help: the following implementations were found:

             <u32 as std::convert::From<bool>>
             <u32 as std::convert::From<char>>
             <u32 as std::convert::From<std::net::Ipv4Addr>>
             <u32 as std::convert::From<std::num::NonZeroU32>>

           and 2 others

   = note: required by `std::convert::From::from`

Errors like this should point to the field