Improve TraitMismatch diagnostic message.

[ilyalesokhin-starkware]

This change is 

[orizi]

Reviewed 1 of 3 files at r1, all commit messages.
Reviewable status: 1 of 3 files reviewed, 1 unresolved discussion (waiting on @ilyalesokhin-starkware)

---

crates/cairo-lang-semantic/src/diagnostic.rs line 512 at r1 (raw file):

                    actual_trt.debug(db),
                )
            }

Suggestion:

            SemanticDiagnosticKind::TraitMismatch { expected_trt, actual_trt } => {
                format!(
                    "Expected an impl of `{:?}`. Got an impl of `{:?}`.",
                    expected_trt.debug(db),
                    actual_trt.debug(db),
                )
            }

[ilyalesokhin-starkware]

Reviewable status: 1 of 3 files reviewed, 1 unresolved discussion (waiting on @orizi)

---

crates/cairo-lang-semantic/src/diagnostic.rs line 512 at r1 (raw file):

                    actual_trt.debug(db),
                )
            }

Done.

[orizi]

Reviewed 2 of 2 files at r2, all commit messages.
Reviewable status: complete! all files reviewed, all discussions resolved (waiting on @ilyalesokhin-starkware)