Hide line numbers in $WORKSPACE errors

[dtolnay]

Closes #83.

Before:

error[E0277]: the trait bound `Ret: serde::ser::Serialize` is not satisfied
   --> $DIR/bad_endpoint6.rs:22:8
    |
22  |     Ok(HttpResponseOkObject(Ret { "Oxide".to_string(), 0x1de }))
    |        ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^ the trait `serde::ser::Serialize` is not implemented for `Ret`
    |
   ::: $WORKSPACE/dropshot/src/handler.rs:846:21
    |
846 |     T: JsonSchema + Serialize + Send + Sync + 'static,
    |                     --------- required by this bound in `dropshot::handler::HttpResponseOkObject`

After:

error[E0277]: the trait bound `Ret: serde::ser::Serialize` is not satisfied
   --> $DIR/bad_endpoint6.rs:22:8
    |
22  |     Ok(HttpResponseOkObject(Ret { "Oxide".to_string(), 0x1de }))
    |        ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^ the trait `serde::ser::Serialize` is not implemented for `Ret`
    |
   ::: $WORKSPACE/dropshot/src/handler.rs
    |
    |     T: JsonSchema + Serialize + Send + Sync + 'static,
    |                     --------- required by this bound in `dropshot::handler::HttpResponseOkObject`

I think compiletest-rs does this by subbing all line numbers with :LL:COL (https://github.com/rust-lang/rust/blob/18f3be7704a4ec7976fcd1272c728974243d29bd/src/test/ui/consts/offset_ub.stderr). I am not a fan of that since I find it breaks the immersion and makes it harder for a human reader to visually perceive the file as a user-facing compiler diagnostic that's worth critically evaluating and improving, as opposed to a test.

[ahl]

Thanks! This is great!