quickcheck!: accept mut bindings in property arguments

[chiliec]

Fixes #269

The quickcheck! macro matched each property argument as $arg_name:ident : $arg_ty:ty, so a mut binding like fn prop(mut xs: Vec<usize>) -> bool failed with no rules expected the token xs``.

Fix: match one or more idents before the colon ($($arg_name:ident)+ : $arg_ty:ty) and forward them verbatim into the generated prop fn. Plain ident: ty arguments expand exactly as before; the fn(...) cast still uses only the types. (A pat_param fragment isn't an option here — macro_rules doesn't allow : after it.)

Tests: added prop_macro_mut_arg(mut xs: Vec<usize>) to the existing quickcheck! { ... } block in src/tests.rs.

Validation (rustc 1.98.1, local):

• With the test but the old macro: cargo test --lib prop_macro → error: no rules expected xs`` (reproduces the issue).
• With the fix: cargo test → 70 passed (lib) + 4 passed (quickcheck_macros), 0 failed.
• cargo fmt --check clean.