Prefer using core crate over std in macro expansions wherever a symbol has
a stable equivalent in core.  This makes it easier to eventually support
no_std in the future once error_in_core stabilises.

    warning: unnecessary hashes around raw string literal
      --> build.rs:10:21
       |
    10 |   const PROBE: &str = r#"
       |  _____________________^
    11 | |     #![feature(error_generic_member_access)]
    12 | |
    13 | |     use std::error::{Error, Request};
    ...  |
    35 | |     }
    36 | | "#;
       | |__^
       |
       = help: for further information visit https://rust-lang.github.io/rust-clippy/master/index.html#needless_raw_string_hashes
       = note: `-W clippy::needless-raw-string-hashes` implied by `-W clippy::all`
       = help: to override `-W clippy::all` add `#[allow(clippy::needless_raw_string_hashes)]`
    help: remove all the hashes around the literal
       |
    10 ~ const PROBE: &str = r"
    11 |     #![feature(error_generic_member_access)]
     ...
    35 |     }
    36 ~ ";
       |

Change span of `as_dyn_error()` to point compile error at attribute.

`rustfmt tests/ui/*.rs`

    warning: item name ends with its containing module's name
     --> impl/src/span.rs:4:11
      |
    4 | pub trait MemberSpan {
      |           ^^^^^^^^^^
      |
      = help: for further information visit https://rust-lang.github.io/rust-clippy/master/index.html#module_name_repetitions
      = note: `-W clippy::module-name-repetitions` implied by `-W clippy::pedantic`
      = help: to override `-W clippy::pedantic` add `#[allow(clippy::module_name_repetitions)]`