Fix #107: support generic functions with reference type parameters in func!

[mazong1123]

Summary

Fixes #107 by removing the type coercion in func! macro's Case 1 and Case 2, which fails for generic functions whose type parameters are reference types.

Problem

When mocking a generic function like nix::unistd::execvp<S: AsRef<CStr>>, the user specifies:

func!(nix::unistd::execvp, fn(&CStr, &[&CStr]) -> Result<...>)

The explicit fn pointer type fn(&CStr, &[&CStr]) desugars to for<'a, 'b, 'c> fn(&'a CStr, &'b [&'c CStr]) (3 independent lifetimes). But the monomorphized execvp::<&CStr> has for<'a, 'b> fn(&'a CStr, &'b [&CStr]) (2 lifetimes — the inner reference's lifetime is tied to the generic parameter). Rust's type system rejects this coercion.

Fix

Replace the type coercion (let fn_val: $fn_type = $f) with a direct cast to *const () and use type_name::<$fn_type>() for the signature string. This:

• Avoids the lifetime coercion entirely
• Produces identical signature strings (both are fn pointer type names)
• Preserves TypeId-based type safety checking
• Is fully backward compatible — all 132 tests pass

For generic functions with reference type params, users provide turbofish:

func!(execvp::<&CStr>, fn(&CStr, &[&CStr]) -> Result<...>)

Testing

• Added test_will_execute_generic_func_with_ref_type_param_via_turbofish — reproduces the exact issue func! macro lifetime mismatch for generic functions #107 pattern
• All 132 tests pass (131 existing + 1 new), clippy clean

Closes #107