macros: avoid trivial cast in #[quickcheck] expansion

[rome-xi]

Closes #261.

#[quickcheck] expanded fn_item as fn(..) which trips trivial_casts when the attribute is applied inside macro_rules that substitutes a type. Coerce through a let binding instead of as, and add a regression test that denies trivial_casts.

[hlop3z]

Verified the regression test gates the bug: with lib.rs reverted to the pre-fix #name as #fn_type expansion (keeping this PR's test and [[test]] entry), cargo test -p quickcheck_macros fails with the same trivial cast error as #261; with the PR applied it passes. (rustc 1.97.0)

One note for reviewers: the Cargo.toml addition isn't incidental — this crate sets autotests = false, so without the explicit [[test]] block, tests/macro_type_subst.rs compiles but silently never runs. Confirmed by removing it.