Better spanning on generated code

[hashedone]

We need to use span pointing more. If something goes wrong with this generated function compilation, the compilation error would point to #[contract], while the function is generally generated from the reply function - I think it would be nice to add the proper span for the whole entity with quote_spanned:

quote_spanned! {
  // Where `reply_span` is a span of the whole reply signature
  reply_span=>
    #[#sylvia ::cw_std::entry_point]
    // ...
}

Note that quote_spaned maintains spans of spanned entities you expand (eg if something is wrong with compiling #custom_msg in resp type, error about that would actually point to the ident we created it from.

Originally posted by @hashedone in #156 (comment)

[hashedone]

First do #98